Watershed Protection and Management

Module 3: Impacts of Urbanization on Watersheds

Topics

I. Land-Use Changes in the Watershed

II. Local Land-Use Decisions and the Site-Development Process

III. Hydrologic, Physical, Water Quality, and Biological Impacts of Urbanization on Streams

I. Land-Use Changes in the Watershed

Before colonization, the Industrial Revolution, and the urbanization of the American landscape, much of the land in the United States was probably forested. What was not forested consisted of grasslands, desert, or tundra with a network of aquatic systems including wetlands, streams, lakes, and other water bodies. Human activities on land have dramatically altered this landscape. Approximately 297 million acres of U.S. forests are estimated to have been converted for agricultural uses in the past 400 years, and approximately 600 million acres were converted for other uses between 1850 and 1900 (Smith & Darr, 2004). Figure 3.1 provides a current picture of land use in the United States. As of 2002, half the land area in the United States is considered rural, 21 percent is forest, and 6 percent is urban land (NRCS, 2003).

Figure 3.1 Land Use/Cover Distribution in the United States (2003)

Source: Adapted from NRCS, 2003

Rural land uses include agricultural uses such as cropland, pasture, and rangelands. Agricultural uses are considered to be the single largest source of impairments to U.S. rivers and streams (EPA, 2000). The processes of clearing and tilling expose soils to runoff, causing erosion and sedimentation (see figure 3.2). Growing crops also involves the use of fertilizers and pesticides, which leach into groundwater and run off into local streams. Stream networks are often disturbed for agricultural uses to reroute water or withdraw groundwater for irrigation. Pasture land can also affect water quality because livestock cause soil compaction, trampling of vegetation, soil erosion, and increased bacteria loads (see figure 3.3). Although considerable strides have been made recently in capping or reducing sediment and nutrient loads from agricultural land, these lands are still considered a significant source of pollution and water quality problems in many urban watersheds, likely because they make up such a vast proportion of land acreage in the country.

Figure 3.2 Exposed Soils Are Susceptible to Erosion from Wind and Rain

Photo courtesy of U.S. Department of Agriculture Photo Gallery, http://photogallery.nrcs.usda.gov/

 

Figure 3.3 Livestock Can Contribute to Bank Erosion and Affect Water Quality

Photo courtesy of the Center for Watershed Protection; used with permission

Although forest land is generally considered to be a land cover that contributes little pollution to our waters (in fact, forests help to reduce pollution), certain land-use activities that take place on forested land may cause pollution.

Silviculture is the practice of managing trees to meet a full range of human activities, including logging, reforestation, and clear-cutting. Some of the potential impacts of silviculture include soil exposure, vegetation removal, and buffer encroachment. Soil exposure and vegetation removal can cause erosion and sedimentation, particularly along slopes and logging roads. Vegetation removal can have a direct impact on animal and plant diversity and abundance.

Mining and other natural resource extraction activities also have serious effects on water resources. Soil exposure, vegetation removal, habitat disturbance, and acid mine drainage are some of the potential impacts. As a result of these activities, erosion and sedimentation can occur, as well as the runoff of copper, arsenic, zinc, lead, mercury, and acids to local streams.

In contrast to rural land, urban land makes up only a small proportion of the total land area in the United States; however, its impacts are still significant. EPA (2000) cites runoff from urban land as the leading source of impairment (along with agricultural runoff) for surface waters nationwide. Urbanization refers to increases in the human population within a specified area accompanied by the construction of buildings, roads, and other infrastructure necessary to accommodate the growth. Urbanization can be measured using a variety of different indicators, such as the percentage of impervious cover, percentage of urban land, population density, and road density. Conversion of land for urban use is characterized by the following progressively transformative alterations on the land (Schueler, 2004):

· conversion to impervious cover

· construction of sewer, water, and stormwater infrastructure

· increase in turf cover

· fragmentation of natural areas

· interruption of the stream corridor

· encroachment and expansion on the flood plain

· increased population density

· increased density of stormwater hotspots

These alterations are briefly described below and in more detail in Schueler (2004).

Conversion to Impervious Cover

The development cycle begins with the clearing of forests, farms, and wetlands, which are replaced by rooftops, roads, parking lots, and other forms of impervious cover. Look at figure 3.4 and move your mouse over the diagram to see several examples of impervious cover.

Impervious cover is any surface that does not allow the natural infiltration of water. It fundamentally alters the hydrology of urban watersheds by generating increased stormwater runoff and reducing the amount of rainfall that soaks into the ground. Measuring impervious cover is the best way to determine the intensity of subwatershed development and predict the severity of impacts to the remaining stream network (CWP, 2003). The relationship between impervious cover and stream health is discussed in section III of this module: Hydrologic, Physical, Water Quality, and Biological Impacts of Urbanization on Streams.

Figure 3.4 Impervious Surfaces in a Watershed

Directions: Place your mouse over the colored squares to highlight the corresponding areas in the photograph.

Source: Center for Watershed Protection, 1999

Construction of Sewer, Water, and Stormwater Infrastructure

In urbanized areas, the aquatic network of streams, wetlands, and other water bodies has been transformed, replaced, or otherwise affected by a complex network of underground pipes and drains. The mixture of sanitary sewer pipes, water pipes, and storm drainpipes can constrain restoration practices, become sources of leaks and overflows, act as fish barriers, capture groundwater (reducing stream flow), reduce pollutant-removal effectiveness of streams, and cause severe erosion from outfalls. Construction of infrastructure can also encourage more development (e.g., sewer line extensions). This may be a desired outcome in some communities, but it can be highly controversial in others.

Increase in Turf Cover

Turf cover includes public and private lawns, cemeteries, golf courses, schools, and parks. Private lawns alone are thought to occupy an estimated 25–40 million acres in the United States (Robbins & Birkenholtz, 2003). Although turf is probably the least studied of all land cover types, it is an ever-growing component of the landscape in the United States.

Most turf areas are continuously mowed and intensively managed for aesthetic purposes. This includes the application of fertilizer and pesticides, often at a much higher rate and frequency than necessary. Studies have shown that pesticide concentrations in urban streams are higher than in agricultural streams (CWP, 2003). Some other impacts of increased turf cover include reduced varieties of plant and animal species, increased human exposure to chemicals, increased water demand, and increased air pollution from lawn mowers (Robbins & Birkenholtz, 2003). In addition, turf tends to be more compacted than native soils and thus has a lower infiltration rate and a higher runoff rate than other land uses. Turf loses 30 percent more water through evapotranspiration than forest land (CWP). From a practical standpoint, the hydrology of many turf areas is more similar to impervious areas than natural ones.

Fragmentation of Natural Areas

In addition to total acreage of forests, wetlands, and other natural areas, consolidation of these areas is also an important consideration in providing adequate habitat and maintaining hydrologic balance. As you learned in module 2, large tracts of contiguous habitat and corridors with high connectivity are the most valuable for wildlife. As land is developed in a watershed, parcel by parcel, these formerly contiguous natural areas are divided into smaller and smaller fragmented patches, reducing connectivity and increasing the edge-to-interior habitat ratio. Invasive plant species, birds, and other exotics often dominate edge habitats.

Interruption of the Stream Corridor

The stream corridor, including the stream and its floodplain, is a critical component in the hydrologic balance in the watershed. With increased urbanization, the stream corridor becomes interrupted more frequently. This means that the continuous stream buffer is more frequently crossed by roads, utilities, dams, or other structures. Continuously flowing streams also are frequently straightened, piped, armored, or otherwise "improved." Some of these "improvements" are designed to keep the stream flow within its banks and prevent erosion or flooding, but they also have a direct impact on stream quality. Stream interruption is an important factor in determining fish passage, channel erosion, and aquatic habitat suitability. Figure 3.5 illustrates how the stream network changes with watershed urbanization.

Figure 3.5 The Stream Network for Watersheds with  20 Percent, 30 Percent, and 50 Percent Impervious Cover

Note that most of the streams in the watershed with 50 percent impervious cover are piped.

20 percent

30 percent

50 percent

Source: Schueler (2004)

Encroachment and Expansion on the Floodplain

Because of the views that a floodplain can offer, building within it has always been a popular option, but it also runs the risk of flooding and property damage. Floodplain modification occurs to allow development and reduce this risk. The most common modification has been to fill the floodplain with earth to provide a higher platform for buildings. Although the fill may provide local relief to landowners, it also sharply reduces the storage capacity of the floodplain and exacerbates downstream flooding problems.

Other flood-control remedies such as channelization, levees, and ditching produce similar effects. In addition, the frequent stream interruptions and encroachments found in urban watersheds may also reduce the capacity of the floodplain to handle floodwaters. Even if encroachment never occurred, urban floodplains will always expand in response to upstream development. Urban watersheds produce higher peak flooding rates; consequently, urban floodplains must expand to accommodate these higher flows.

Increased Population Density

Urbanization is marked by an increase in population density. Although concentrating population density into defined areas is good from land-use planning and resource-conservation perspectives, the intensity of potential pollutant-producing activities in these areas can also contribute to local pollution. One person's activity over 50 acres may not have a big impact, but 50 people's activities over just one acre can have an impact of serious intensity. Littering, dumping, failing to pick up after pets, applying pesticides, and other activities can adversely affect the watershed. Conversely, planting trees, installing rain barrels, and conducting community cleanups can have a positive effect on the watershed. Thus, the collective behaviors of watershed residents determine whether pollution will be generated or prevented.

Increased Density of Stormwater Hotspots

Stormwater hotspots are commercial, industrial, institutional, municipal, and other land uses that produce high levels of stormwater pollutants and/or present a higher potential risk for spills, leaks, and illicit discharges (Wright, Swann, Cappiella, & Schueler, 2004). Stormwater hotspots can include everything from industrial office parks, wastewater treatment plants, chemical plants, steel plants, textile mills, manufacturing plants, and municipal waste yards, to gas stations, restaurants, laundromats, and strip malls. These land uses have varying degrees of impact on the watershed, depending on the amount and type of pollutants involved and how they are handled, transported, and stored. With urbanization, the density of these hotspots increases, along with the potential for contamination of stormwater by pollutants from these sites.

II. Local Land-Use Decisions and the Site-Development Process

Each year in the United States, more than 1.5 million acres of land are developed (CWP, 1998). In most cases, this occurs as conversion of agricultural or forest land for urban and suburban uses. Most of the decisions regarding these types of land-use changes are not regulated at the federal or even state levels; land use is often determined at the city, county, town, or township levels. These local governments are often called jurisdictions or municipalities. Although the exact structure and powers allotted to these local entities vary widely because they are set by the states, most local governments have power over land-use planning and the local development process.

To better understand how the development process affects watersheds (and how to reduce these impacts), you should know the basics about how land-use planning is implemented and how the land-development process works. This section provides an overview of land-use planning at the local level and discusses the three major elements of the site-development process:

1. site plan submittal, review, and approval

2. the construction process

3. occupancy of the site after construction

These three elements include critical decisions about where to develop, how to develop, and how to manage the land after development.

Land-Use Planning at the Local Level

Because the structure of local governments varies, a variety of departments, commissions, or boards may influence the land-use planning process. Planning departments and elected planning commissions are two examples of entities that commonly play critical roles in making local land-use decisions. Other factors such as the size of the community, local politics, and the interest of citizens will often dictate the local stance on development. For example, development may be desirable for communities interested in attracting more citizens or growing their economic base. Other communities may wish to curb development because they want to preserve community green spaces or special natural resources. Regardless of the local-government structure or local politics, key elements of local land-use planning include zoning, development codes, and comprehensive planning.

Most local jurisdictions specify which land-use activities are allowable in specific zones within the jurisdiction. This process of defining zones and allowable land uses is called zoning, and it is usually specified in a local zoning ordinance. The first zoning ordinance was passed in New York City in 1916; and by the 1930s, most states had adopted zoning laws (Legal Information Institute, 2006).

The local zoning ordinance spells out all the criteria and regulations that apply in each individual zone, such as allowable density and building height and area. Each defined zone is called a zoning category; typical zoning categories include residential, commercial, industrial, transportation, agricultural, and institutional (e.g., schools and government buildings) land uses. These are usually broken down into more specific zoning categories. For example, residential zones are defined by the allowable housing density, so an area that is zoned R-1 means the allowable use is residential with a density of one lot per acre. Figure 3.6 shows an example of a zoning map for the city of New Market, which is in Frederick County, Maryland. Click on the map to see a larger version with details.

Figure 3.6 Zoning Map for the New Market, Maryland, Region

Source: Frederick County, Maryland (2006) http://www.co.frederick.md.us/Planning/CompPlan/NewMarket/BOCC-PR-ZON-NM.pdf)

In addition to the traditional zoning ordinances, jurisdictions will sometimes implement overlay, cluster, mixed-use, or other special zoning. Overlay zoningsuperimposes additional regulations or development criteria on top of the existing zoning. This method is often used to protect sensitive areas, such as steep slopes. The overlay zoning approach allows the local government to impose these additional regulations in specific areas without having to change the zoning ordinance or re-zone all of the areas that would be affected. The criteria in the overlay zone supersede any conflicting requirements outlined in the regular zoning ordinance for areas included in the overlay zone. Figure 3.7 illustrates the overlay zoning concept.

Figure 3.7 A Sample Overlay Zoning District

Adapted from Jon Witten, Horsley and Witten

Cluster zoning allows for residential lots to be "clustered" into one section of the parcel while preserving a larger portion of the site as open space. Clustering does not increase the amount of housing, but does increase the density of the space within which development occurs. In contrast, mixed-use zones allow for a mixture of commercial, residential, or other land uses that are typically separated by zoning. One reason that this type of zoning may be desirable is that it could promote less automobile travel and more pedestrian travel if commercial areas are mixed with residential areas. Some other zoning types include environmental preservation zones, planned unit developments, and conservation zones. Depending on the community, each zoning category may be used and applied differently.

Comprehensive planning determines where future growth will occur and results in a plan that is usually updated every 5 to 10 years. Comprehensive plans, also known as master plans or general plans, guide the placement of infrastructure such as roads, sewer and water lines, and schools. These plans can be used to define urban growth boundaries and take into account the long-term potential for growth and development in the community.

Not all communities conduct comprehensive planning, but communities that write comprehensive plans are usually required by the state to do so. Some common elements of a comprehensive plan include community visioning, land-use regulations, transportation guidance, infrastructure plans (including housing, utilities, stormwater master plans, sewer plans, and water supply), public facilities plans, natural resource management plans, and open space plans (including greenways and recreational space).

Although zoning ordinances and comprehensive plans determine where development will occur, several other types of local regulations govern what the resulting development will look like. These regulations are found in the form of codes, ordinances, and design manuals and may relate to the following elements:

· construction of streets and drainage structures

· subdivision design

· landscaping

· stream buffers/floodplains

· building codes

· septic systems

· parking lot design

· signs

· utilities

· forest conservation

· wetland protection

· stormwater management

Many of the above ordinances set criteria for individual parcels or lots regarding density, building size and height, landscaping, side and front yard setbacks, road frontages, driveway width, sidewalks, road width, curbs and gutters, the number and size of parking spaces, and septic system design and location. Figure 3.8 illustrates some of the typical criteria that apply for a residential lot. Hold your mouse over the diagram to see more information.

Figure 3.8 Typical Criteria for a Residential Lot

Source: Schueler (1995)

The setbacks show the portions of the lot where buildings and pavement cannot be located. So, if the developer decided to place the house farther back on the lot than its current location, he could move it only as far back as the rear yard setback line to meet the required setbacks. Similarly, if the homeowner wanted to attach a side addition in the future, the addition could extend out only as far as the side yard setback line. With a one-acre lot, it would be difficult to build out the lot so that the buildings and pavement filled the entire lot, right up to each of the setbacks (although some of the "McMansions" out there could probably do it!). The maximum building footprint criteria (5 percent of the lot) is actually designed to prevent this type of situation.

When a new development is proposed, a person who wishes to develop a parcel of land must first submit a site plan. Site plans are required to determine whether a development meets a community's development codes, ordinances, and standards. The typical process for site plan submittal, review, and approval is described below. This is not necessarily the same procedure applied for redevelopment, road construction, or other unique projects. In addition, commercial site reviews may differ significantly from residential ones.

Site Plan Submittal, Review, and Approval

The ideal site plan review process will include a concept plan stage, during which the permit applicant (e.g., a developer) meets with the approval authority (e.g., the local planning department) to check for code compliance before the applicant makes any significant investments. Elements that are usually included in a concept plan include:

· location map

· parcel boundaries

· streets, alleys, parking lots, building footprints

· soil map

· resource protection areas

· stormwater management plan

After the concept plan stage, the applicant must obtain any permits required by nonlocal authorities, such as the EPA or Army Corps of Engineers. An example of a nonlocal permit that might be required is a Section 404 permit for impacts to wetlands. Section 404 of the Clean Water Act requires a permit for deposits of dredge or fill material into U.S. waters, including wetlands. Therefore, if the site includes a wetland, an official delineation of its boundaries must be completed and a Section 404 permit obtained for any proposed impacts before a formal site plan is submitted. Other nonlocal permits may require environmental-impact statements and, for large projects, public hearings due to transportation issues and the controversial nature of some developments.

Next, a formal site plan is submitted to the local approval authority for review. Because so many different codes and ordinances regulate development, various departments may also review the plan. The reviewers are responsible for determining what, if any, changes must be made to the plan to conform to local, state, and federal regulations. Table 3.1 lists the local departments that may review site plans and the elements of the site plan they typically review.

Table 3.1 Local Site Plan Review Responsibility

Department	Review Responsibility
Planning and Zoning	site plans
Environmental Division	erosion and sediment control, stormwater, wetland protection
County Engineer	private roads, conservation easements
Landscape Planner	landscape plan
Sewer Authority	utility plans
Fire Department	fire protection enforcement
Department of Transportation	public roads, traffic analysis
Health Department	utility plans, septic systems

The formal site plan amplifies the detail provided in the approved concept plan. Applicants must show evidence that they have:

· secured environmental permits (e.g., section 404 permits)

· made legal arrangements for future management of common areas

· paid the appropriate fees for plan review and inspection

The Construction Process

The construction process involves a number of different contractors, each of whom deals with a different portion of the construction, such as clearing and grading, environmental management, paving, utilities, building construction, and landscaping. All parties involved must be aware of and follow the development regulations. Ideally, during construction, the process includes marking any natural areas to be preserved on the site, installing erosion and sediment practices, and then commencing with the clearing and grading phase. Landscaping is usually the last major portion of the construction process.

Throughout the construction process, environmental criteria must be met to reduce potential impacts to natural resources. These criteria are set forth in forest conservation, erosion and sediment control, wetland protection, stream buffer, and stormwater management regulations. Ideally, the local government will inspect the site during construction to ensure that approved plans are actually constructed to meet environmental and other requirements. For example, the EPA's NPDES regulations require that regulated sites be inspected every 14 days to ensure erosion and sediment control practices are installed, maintained, and in good working condition.

Not all communities have environmental regulations, but even where they do exist, impacts to the land may still occur during the construction process. These impacts include loss of native vegetation, removal of topsoil, soil compaction, alteration of natural drainage patterns, soil erosion and transport into local waters, and creation of new impervious cover, which increases the volume of runoff that leaves the site and reaches local waters. At many development sites, the entire parcel is cleared to allow access for construction equipment, install drainage infrastructure, and make building more efficient. Although it is not always necessary to clear the entire site to build lots, this has become a standard practice in many locations (see figure 3.9). Notable exceptions include communities that have forest-conservation or open-space requirements.

Next, the site is graded, meaning that the natural topography is altered to accommodate the placement of roads and buildings and to direct runoff away from these areas. Grading includes "cut and fill," during which large amounts of soil are removed or cut from one portion of the site and used to fill in another area to bring it up to the desired elevation. Soils take many years to form and consist of various layers, including a fertile topsoil layer that contains organic matter and nutrients. When cut and fill activities occur, this topsoil is often removed or buried, thus reducing the fertility of the resulting soil.

Figure 3.9 Mass Clearing, Grading, and Removal of Topsoil at a Construction Site

Photo courtesy of the Center for Watershed Protection; used with permission

Grading activities also result in soil compaction from heavy-equipment use. In fact, some portions of the site are intentionally compacted to meet engineering standards for bearing structures or traffic loads. Soil compaction is frequently evaluated by measuring the soil bulk density. Bulk density is defined as the mass of dry soil divided by its volume and is expressed in units of grams per cubic centimeter (g/cc) (Schueler, 2000).

Bulk density is a useful indicator of the structure of a soil and can help predict its infiltration rate and water-holding capacity. In general, as bulk density increases, its infiltration rate decreases, and a greater proportion of the rain that falls on it is converted to runoff. The surface bulk density of most undisturbed soils ranges from 1.1 g/cc to 1.4 g/cc, while many urban soils have bulk density of 1.5 g/cc or greater (Schueler, 2000). Compacted soils not only produce more runoff, but they also make it difficult for vegetation to grow.

The clearing and grading process also alters the natural hydrology of a site and may include filling in depressions and relocating streams. The addition of impervious cover greatly increases the amount of runoff generated at the site. Site designers usually try to move this water off the site as quickly as possible. This means putting runoff into storm drainage pipes and installing curbs and gutters along streets. The runoff is then moved swiftly and efficiently off the site into the storm drainage system, which in most cases ultimately discharges into a local stream, wetland, lake, or river (see figure 3.10). The impacts of increased runoff on receiving waters are discussed later in this module.

Figure 3.10 Stormwater Drainage from a Development

Based on Mellis & Schuster (2001)

Post-Construction Occupancy

After construction is complete, a final inspection is conducted to determine whether the site meets all the code requirements. A local government that has an effective environmental management program will inspect stormwater management practices to ensure that they have been installed and are functioning properly. If the site passes inspection, an occupancy permit is issued. In a best-case scenario, the reviewing authority will issue an occupancy permit based on the following criteria:

· temporary sediment controls have been removed

· stormwater treatment practices are in good working order

· permanent vegetative cover is on all exposed areas

· any damage to resource-protection areas and buffers has been corrected

· as-built plans have been submitted

The reviewing authority should not issue an occupancy permit until the builder has met all obligations.

A final note of consideration in the land-use and development process is that although the construction process itself is potentially the most damaging phase, post-construction activities may continue to contribute additional impacts. Activities such as turf management, spills and illegal dumping, and improper maintenance of septic systems are a few examples. Section III describes the specific ways that streams respond to these changes caused by urbanization.

III. Hydrologic, Physical, Water Quality, and Biological Impacts of Urbanization on Streams

Impacts to streams in urban and urbanizing watersheds can be directly evaluated through various field studies of in-stream habitat condition, stream hydrology, aquatic wildlife diversity, and other factors. In lieu of detailed field assessments, a simple method to remotely estimate the impacts of urbanization on local streams is to apply the Impervious Cover Model (ICM) (CWP, 2003). The ICM predicts potential stream quality based on the amount of impervious cover in a given watershed. The ICM is based on a review of more than 225 studies, about 50 of which directly support the relationship between watershed impervious cover and overall stream quality (CWP). The ICM predicts that most stream-quality indicators decline when watershed impervious cover exceeds 10 percent, with severe degradation expected beyond 25 percent (CWP). Figure 3.11 provides a graphical representation of the ICM.

Figure 3.11 The Impervious Cover Model

Source: CWP (2003)

The ICM thresholds of 10 percent and 25 percent produce three categories by which to classify stream condition in a watershed (or individual subwatersheds). These categories include:

Table 3.2 Watershed Categories and Corresponding Impervious Cover Model Thresholds

Watershed Category	ICM Threshold
sensitive	< 10% impervious cover
impacted	10–25% impervious cover
nonsupporting	> 25% impervious cover

Streams found in sensitive watersheds can be characterized by such features as high water quality, pristine stream channels, diversity and numbers of aquatic species, mature forest cover buffers, and/or large quantities of large woody debris (CWP, 2003) (see figure 3.12).

Figure 3.12 A High-Quality Section of Stream in Yarmouth Creek, Virginia, Considered Sensitive

Photo courtesy of the Center for Watershed Protection; used with permission

In impacted watersheds, evidence of hydrologic, physical, and water quality impacts are generally observed. Typical characteristics of an impacted stream may include some stream incision, stream erosion, a reduced number of aquatic species, and reduced forest buffer cover (CWP, 2003).

In nonsupporting watersheds, streams often are characterized by reduced biodiversity and aquatic species abundance, severe stream incision and downcutting, increased peak floods, and reduced forested riparian buffers (CWP, 2003). These streams may also be characterized by water quality problems such as bacteria, pathogens, phosphorous, nitrogen, metals, and other pollutants, all of which constitute an aquatic environment that is not conducive to supporting aquatic life (CWP).

Several caveats exist to the ICM:

· It predicts rather than prescribes stream quality (CWP, 2003).

· Although the ICM works for first-, second-, and third-order streams, it has not been tested for larger rivers and other water bodies such as lakes, aquifers, and estuaries (CWP, 2003).

· The ICM does not predict the precise response of an individual stream-quality indicator, but rather predicts the average behavior of a group of indicators over a range of impervious cover (CWP, 2003).

· The strength in the relationship of impervious cover to predict stream quality actually increases with increased impervious cover (CWP, 2003). For example, a sensitive stream (< 10 percent IC) may have surprisingly poor water quality because of the influence of other watershed factors, such as extensive agricultural uses (see figure 3.13; hold your mouse over the figure for more information). In these instances, impervious cover may not be the best indicator of stream quality; and other indicators, such as riparian forest cover, active cropland, or density of livestock, are probably better indicators of watershed health.

Figure 3.13 Eroded Banks at a Cow Crossing Where Impervious Cover Is Less Than 10 Percent

Source: Center for Watershed Protection; used with permission

In a review of data to support the ICM, 26 stream-quality indicators were found to have a direct correlation with development in a watershed. These are listed in table 3.3 and are categorized as hydrologic, physical, water-quality, or biological indicators. The remainder of section III describes the hydrologic, physical, water quality, and biological responses of streams to urbanization.

Table 3.3 Stream-Quality Indicators Linked to Watershed Development

Hydrologic Indicators

increased runoff volume

increased peak discharge

increased frequency of bankfull flow

diminished baseflow

Physical Indicators

stream channel enlargement

increased channel modification

loss of riparian continuity

reduced large woody debris

decline in stream habitat quality

changes in pool/riffle structure

reduced channel sinuosity

decline in streambed quality

increased stream temperature

increased number of road crossings

Water-Quality Indicators

increased nutrient load

increased sediment load

increased metals and hydrocarbons

increased pesticide levels

increased chloride levels

violations of bacteria standards

Biological Indicators

decline in aquatic insect diversity

decline in fish diversity

loss of coldwater fish species

reduced fish spawning

decline in wetland plant diversity

decline in amphibian community

Adapted from Center for Watershed Protection (2003)

Hydrologic Impacts

As watersheds are developed, the addition of impervious surfaces, compaction of soils, and construction of stormwater drainage systems that deliver water more quickly to the stream all contribute to changes in stream hydrology. These changes include increased runoff volume, increased peak discharge, increased frequency of bankfull flow, and diminished baseflow (CWP, 2003). Each is described below.

Increased Runoff Volume

During development, as surfaces are converted from pervious to impervious, a greater proportion of rain that falls on these surfaces is converted to runoff. As you learned in module 2, runoff volume is directly related to watershed impervious cover. The increase in runoff volume with urbanization is clearly shown when comparing the runoff volume between a parking lot and a meadow. Schueler (1994) estimated the runoff volume from a one-inch storm to be 3,450 cubic feet for a parking lot and 218 cubic feet for a meadow. As you can see, the parking lot runoff is more than 15 times the volume of runoff from the meadow. The cumulative increase in runoff volume from paved areas throughout an urban watershed directly translates into increases in the peak discharge of local streams.

Increased Peak Discharge

The peak discharge rate is defined as the maximum rate of flow during a storm event. Peak discharge rates are often used to determine flood risk. Watershed development has a strong influence on both the magnitude and frequency of flooding in urban streams (CWP, 2003). Several studies of urban areas have shown that peak discharge is higher in urban areas as compared to rural areas, with peak discharge rates for the 1-year storm increasing by a factor of ten, for the 10-year storm by a factor of 2 to 3, and for the 100-year storm by a factor of 2 to 2.5 (CWP). The increase in peak discharge rates following urbanization shifts the elevation of the 100-year floodplain upward, which may put more property and structures at risk (see figure 3.14).

Figure 3.14 Changes in Peak Discharge Rates Shift the Floodplain After Development

Based on Schueler (1987)

Increased Frequency of Bankfull Flow

The magnitude, frequency, and duration of bankfull flows and the flashiness of flows also increase in urbanized areas. Bankfull flows are defined as the stream discharge corresponding to the flood elevation that first overtops the banks of stream channels. Bankfull flows are channel-forming events because they have the most impact on channel shape and size, and in most streams equate to the 1.5- to 2-year storm (Leopold, Wolman, & Miller, 1964; Anderson, 1970). How often these bankfull events occur and how much they are exceeded can have an impact on sediment transport and stream-channel enlargement. Flashiness is defined as the percent of daily flows each year that exceeds the mean annual flow (CWP, 2000). As expected, urban streams show more flashiness (CWP, 2003).

Diminished Baseflow

One might expect that because less water is infiltrated in urbanized areas, there would also be decreased baseflow during dry periods. Several studies do link decreased baseflow with increased watershed development; however, other studies have shown that leaking sewers and water pipes in urban areas may lead to inconclusive results (CWP, 2003). In fact, in arid and semi-arid areas, increased flow from lawn watering and sewage treatment plants was actually shown to increase baseflow (Caraco, 2000a).

The changes in watershed and stream hydrology described here have impacts on the physical structure of streams and ultimately affect the water quality and biological diversity of streams, wetlands, and other waters. These impacts are described below.

Physical Impacts

As noted above, urbanization increases the frequency and duration of the bankfull, or channel-forming flows in streams. This in turn creates a cycle of active channel erosion and greater sediment transport in streams. The stream channel erodes because it must adjust to its increased flow and sediment supply. In addition, changes in stream hydrology and sediment loads affect the quality of instream habitat. Other physical changes to the structure of stream from urbanization include increased temperature. Each is described below.

Channel Widening and Downcutting

Stream channels typically expand their cross-sectional area to accommodate the increased flows and sediment loads brought on by urbanization. This expansion occurs through channel widening, downcutting, or both. Channel widening is the lateral expansion of the stream bank and its floodplain, and channel downcutting, also called incision, is when a stream cuts down into the stream bed (Allen & Narramore, 1985; Booth, 1990; Morisawa & LaFlure, 1979).

Stream beds with underlying bedrock usually undergo channel widening because the bedrock impedes downcutting. Channel widening is also more likely to occur where grade controls, such as culverts and road crossings, exist (CWP, 2003). Channel incision is affected by the type of substrate present, with less occurring in bedrock and cobble areas and more occurring in stream banks with sand, gravel, and clay substrates (Booth; Allen & Narramore). In addition, grade-control features such as pipelines, bridges, and culverts impede incision (CWP).

Research shows that stream cross-sectional areas can enlarge as much as two to eight times after urbanization (Pizzuto, Hession, & McBride, 2000; Caraco, 2000b; Hammer, 1972). Stream channel widening and downcutting are accomplished through the erosion of stream beds and banks, which contributes a greater supply of sediment to the stream. Figure 3.15 illustrates the change in a stream channel cross section over time with increasing urbanization in Watts Branch, Maryland. Hold your mouse over the figure to get more information.

Figure 3.15 Change in a Cross Section of Watts Branch Stream Channel over Time

Sources: Figure: Adapted from Center for Watershed Protection; Macris, Hendricks, and  Glascock, Inc.; and Environmental Systems Analysis, Inc. (2001), pp. 2–4 Statistics: Caraco (2000c), pp. 99–104

Decline in Stream Habitat Quality

Urbanization is also associated with a decline in stream habitat quality. Stream habitat quality is often measured using protocols that combine a multiple number of habitat factors, such as substrate quality, riffle frequency, and sediment deposition, into a single score. Several studies report correlations between increased levels of impervious cover and habitat scores (Black & Veatch, 1994; Booth & Jackson, 1997; Hicks & Larson, 1997; Maxted & Shaver, 1997; Morse, 2001; Stranko & Rodney, 2001).

Other studies have evaluated the effect of urbanization on individual measures of stream habitat, such as bank stability, large woody debris, and embeddedness. Embeddedness is the extent to which the rock surfaces found on the stream bottom are filled in with sand, silts, and clay (CWP, 2003). Because the stream bottom is important habitat for aquatic insects and spawning fish, embeddedness is a critical measure of stream health. In a healthy stream, the spaces between the rocks lack fine sediments and have low embeddedness. Research has shown that urban streams are usually highly embedded, have less large woody debris, and have more unstable channels (CWP).

Increase in Stream Temperature

Increases in stream temperature are also associated with urbanization because of the conversion of pervious areas to impervious surfaces, such as concrete and asphalt. Impervious surfaces absorb energy from the sun, often reaching temperatures of 110º to 120º F (CWP, 2003). The urban ecosystem loses the natural cooling process of trees and gains heat from the combustion of fossil fuels, resulting in warmer temperatures (e.g., the urban heat island effect). When a rain event occurs on a hot, sunny day, the stormwater that runs off these hot surfaces is often much warmer than aquatic species can tolerate. Galli (1990) found that summer stream temperatures increased by 5º to 12º F in response to an increase in impervious cover.

Water-Quality Impacts

Although the quantity of stormwater that is "dumped" into local streams from urban areas increases dramatically after a storm, the quality of this water is also a big concern. Stormwater washes pollutants from impervious surfaces, turf areas, and other point and nonpoint sources into the local streams. Depending on the land use, pollutants encountered may include sediment, nutrients, trace metals, hydrocarbons, bacteria, pathogens, organic carbon, MTBE, pesticides, or deicers. Concentrations of these pollutants in urban streams are often significantly higher than in streams found in undeveloped watersheds. Each is discussed below.

Sediment

Sediment affects streams in several ways:

· Deposits of sediment can smother habitat for aquatic insects, fish, and freshwater mussels and decrease channel flow capacity, contributing to flooding.

· Increased sediment loads can contribute to increased stream temperature and depletion of dissolved oxygen.

· Sediment can carry pollutants, such as nutrients and metals, which often bind to sediment particles.

Sources of sediment in urban watersheds include stream bank erosion, exposed soils (e.g., construction sites), and impervious surfaces. Over time, impervious surfaces accumulate dirt and sediment, which are then carried to the stream during a storm event. Construction sites can be a major source of sediment if they lack proper erosion- and sediment-control practices.

Nutrients

Increased inputs of nutrients, such as phosphorus and nitrogen, affect the stream environment by contributing to eutrophication. Eutrophication is the process of over-enrichment of water bodies by nutrients, often typified by the presence of algae blooms (CWP, 2003). The increased nutrient inputs cause excessive algae growth. When these algae eventually die, the decomposition process uses up a lot of oxygen, resulting in depletion of dissolved oxygen and, in extreme cases, massive fish kills. Eutrophication effects have been studied mostly in lakes, reservoirs, and estuaries, to which many small streams eventually drain.

Sources of nutrients in urban runoff include fertilizer, pet waste, organic matter, and stream bank erosion. Atmospheric deposition of nutrients from automobile combustion, power plants, and industry can also be a significant source. One study found that nutrient concentrations in lawn runoff were four times greater than for impervious surface runoff (Bannerman, Owens, Dodds, & Hornewer, 1993; Steuer, Owens, Dodds, & Hornewer, 1997; Waschbusch, Selbig, & Bannerman, 2000). This finding is significant because lawns can account for more than 50 percent of the total area of urban and suburban watersheds (CWP, 2003).

Trace Metals

Trace metals that are commonly found in urban runoff include zinc, copper, lead, cadmium, and chromium. Increased concentrations of these metals in urban runoff can have detrimental effects when this runoff reaches a stream. A major concern is their potential toxicity to aquatic organisms. High concentrations can lead to bioaccumulation of metals in plant and animal tissues, potential chronic or acute toxicity, and contamination of sediments, which affect bottom-dwelling organisms (Masterson & Bannerman, 1994). Several studies show that the mortality rate of aquatic species increases with increased exposure to trace metals (Crunkilton, Kleist, Ramcheck, DeVita, & Villeneuve, 1997; Masterson & Bannerman).

Sources of trace metals include automobiles, atmospheric deposition, rooftops, and urban runoff, with automobiles being the major source. In a 1992 study by Woodward-Clyde Consultants, 50 percent of the copper load in surface waters was attributed to automobile brake pads. Sources of lead include atmospheric deposition, diesel fuel emissions, rooftops, and streets. The predominant sources of zinc are paints, tires (Woodward-Clyde), and galvanized gutters and downspouts. Chromium hotspots include industrial and commercial runoff. All of these sources contribute metals to the urban environment, where they are ultimately washed off into surface waters.

Hydrocarbons

Hydrocarbons are defined as petroleum-based substances; they are made up of oil, grease, and polycyclic-aromatic hydrocarbons (PAH). They attach to sediment or organic carbon and are carried into streams during storm events. The primary concern with hydrocarbons is their potential bioaccumulation and toxicity in aquatic organisms. Although research has documented bioaccumulation in crayfish, clams, and fish (Masterson & Bannerman, 1994; Moring & Rose, 1997; Velinsky & Cummins, 1994), the long-term affects of PAH compounds on aquatic life is still unclear.

Sources of hydrocarbons are plentiful in the urban environment. They include gas stations, parking lots, convenience stores, and streets (Schueler & Shepp, 1993). A 1998 study by Lopes and Dionne found that 64 percent of hydrocarbon loads came from commercial parking lots. High concentrations of PAH are found in coal-based parking lot sealants, which lead to aquatic mortality (Richardson, 2006).

Bacteria and Pathogens

Bacteria are single-cell organisms that are too small to see with the naked eye (CWP, 2003). Three types of bacteria—fecal coliform, fecal streptococci, and Escherichia coli (E. coli)—are consistently found in urban runoff. The presence of these bacteria confirms the existence of sewage or animal wastes in urban runoff and also indicates that other harmful bacteria, viruses, or protozoans may be present. Coliform bacteria are indicators of potential public health risks and are not actual causes of disease. Water quality standards for bacteria are developed to protect human health based on exposure to water during recreation and drinking.

A pathogen is a microbe that is actually known to cause disease under the right conditions (CWP, 2003). Two of the most common waterborne pathogens are Cryptosporidium parvum and Giardia lambia. Cryptosporidium, an intestinal parasite that infects cattle and domestic animals, can be transmitted to humans and cause life-threatening problems. Giardia causes intestinal problems in humans and animals.

Sources of coliform bacteria include waste from animals, including pets, wildlife, and livestock. Bacteria are transported to streams through stormwater that picks up waste from dogs and waterfowl. Indirect sources of bacteria include leaky septic systems, illicit discharges, and sanitary sewer overflows. Sources of pathogens include human sewage and animal feces. In urban areas, animal sources typically include dogs and Canada geese, which are found in large populations in urban and suburban stormwater ponds.

Organic Carbon

Total organic carbon is often used as an indicator of the amount of organic matter in a water sample. Organic carbon has several effects on the environment:

· As the amount of organic carbon in a watershed increases, more oxygen is consumed and less is available for other aquatic life.

· Like sediment, organic carbon can be a carrier for other pollutants.

· Organic carbon is a concern for drinking water quality because it reacts with chlorine to form trihalomethane and other disinfection by-products that are hazardous to human health (U.S. EPA, 1998).

Sources of organic carbon include decaying leaves, organic matter, sediment, and combustion by-products. In an urban watershed, curbs, storm drains, streets, and stream channels all contribute organic carbon to a stream.

MTBE

Methyl tertiary butyl-ether (MTBE) is a volatile organic compound (VOC) that has been added to gasoline since the 1970s to help gasoline burn cleaner. According to the EPA, MTBE is a potential human carcinogen at high doses. It is also a known carcinogen to small mammals and may be toxic to aquatic life in small streams (Delzer, 1996). Because MTBE is highly soluble in water and also causes taste and odor problems, it is a potential threat to the drinking water supply. Sources of MTBE include roads, gas stations, watercraft, vehicle emissions, and leaky underground gasoline storage tanks. Although the use of MTBE has sharply declined since 2000, the extent of its impact on local waters is still largely unknown.

Pesticides

The application of pesticides is common on lawns in urban residential neighborhoods. The EPA estimates that urban lawns receive 70 million pounds of pesticides each year. Pesticides include herbicides that are used to control weeds and insecticides that are used to control insects. Many pesticides are known or potential carcinogens and can be toxic to human and aquatic species, but few studies have evaluated the chronic toxicity of pesticides. Studies have primarily documented the acute toxicity of pesticides such as diazinon.

Sources of pesticides in urban watersheds include residential lawns, golf courses, ball fields, parks, rights-of-way, and pesticide storage areas. Several studies have shown that the frequency and concentrations of insecticides in urban streams is greater than what is found in agricultural streams (USGS, 2001a, 2001b, 1999a, 1999b, 1998; Ferrari et al., 1997).

Deicers

Deicers are used to melt snow and ice from roads and sidewalks. Chlorides (most commonly sodium chloride), urea, and ethylene glycol are the three most common types of deicers. During snowmelt, these chemicals enter streams in high concentrations and have potentially harmful effects:

· Chloride can harm aquatic and terrestrial life and can also contaminate drinking water supplies (Ohrel, 1995).

· Chloride causes taste problems with drinking water and is very difficult to remove once it enters the water.

Source areas for deicers in urban watersheds include roads, parking lots, sidewalks, storm drains, airport runways, and snow-collection areas.

Biological Impacts

As a result of urbanization, changes to the hydrology, physical structure, and water quality of streams can have a significant impact on aquatic biodiversity. Biological impacts to streams are often measured using biological indicators, including the abundance and diversity of indicator species, such as aquatic insects, fish, amphibians, and freshwater mussels. Biological impacts may also include effects on wetland vegetation found in riparian and floodplain areas. The impacts of urbanization on each of these factors are described below.

Aquatic Insect Diversity

The aquatic insect community typically responds to increasing watershed development by losing species diversity and richness and shifting to more pollution-tolerant species (CWP, 2003). Pollutant-sensitive species, such as Ephemeroptera (mayfly), Plecoptera (stonefly), and Tricoptera (caddisfly), are often absent in urban streams and replaced by more pollution-tolerant organisms such as chironomids, amphipods, and snails. Most streams with less than 25 percent impervious cover do not support a diverse aquatic community (CWP). Many studies suggest that aquatic insect diversity declines around a watershed impervious cover of 10 percent (CWP).

Fish Diversity

The fish community typically responds to increasing watershed development with a reduction in total species, loss of sensitive species, a shift toward more pollution-tolerant species, and decreased survival of eggs and larvae (CWP, 2003). Salmonid species (trout and salmon) are particularly sensitive to watershed impervious cover (Luchetti & Feurstenburg, 1993; Steward, 1983). Several studies indicate fish diversity declines around a watershed impervious cover of 10 percent, with some studies showing variability (Boward, Kazyak, Stranko, Hurd, & Prochaska, 1999; Galli, 1994; Klein, 1979; Limburg & Schmidt, 1990; MNCPPC, 2000; MWCOG, 1992; Steward).

Amphibian Diversity

Amphibians spend parts of their life cycles in riparian, wetland, or lakeside environments. They are particularly sensitive to pollution and their numbers and diversity are a good measure of the health of a local stream. Although amphibian response has been studied less than other indicators, reduced amphibian diversity with increasing watershed impervious cover has been shown (Horner et al., 1997). Other studies have linked amphibian diversity with increased water-level fluctuations (WLF) in wetlands due to increased stormwater discharges. Changes in WLF can affect amphibian breeding habitat and the abundance of egg clutches, thus influencing amphibian richness (Chin, 1996).

Freshwater Mussel Diversity

Freshwater mussels are excellent indicators of water quality because they are immobile filter feeders. They are also one of the most threatened species. Two-thirds of the nation's freshwater mussels are at risk of extinction, and almost 1 in 10 may already have vanished forever (EPA, 2006). Williams et al. (1993) concluded that the decline is due to the modification of habitat and sedimentation—sediment from eutrophication and dams/impoundments interferes with their filter feeding and metabolism (Aldridge, Payne, & Miller, 1987). Freshwater mussels are also very sensitive to heavy metals and pesticides (Keller & Zam, 1991), which inhibit their respiratory efficiency and accumulate in their tissues (Watters, 1996). Freshwater mussels have acute sensitivity to pesticides, and that makes them a useful indicator of toxicity in a system.

Wetland Plant Diversity

In response to increased urbanization, the diversity and richness of wetland plant species is reduced (Ehrenfeld, 2000). In addition, sensitive species are replaced by more pollution-tolerant species or invasive species (Ehrenfeld). Other key findings from Wright (2006) include the following:

· Considerable evidence demonstrates that wetland communities are particularly vulnerable to increases in water-level fluctuation caused by excessive stormwater inputs, and that WLF provides more favorable conditions for the spread of invasive plant species.

· Trace metals and hydrocarbons are clearly accumulating in the tissues of plants and animals in urban wetlands exposed to stormwater, although it is unclear whether the reported levels are causing toxicity in the food chain.

· When sediment inputs create deposition in urban wetlands, the structure of the plant community shifts away from sensitive species and toward invasive species.

References

Aldridge, D., Payne, B., & Miller, A. (1987). The effects of intermittent exposure to suspended solids and turbulence on three species of freshwater mussels. Environmental Pollution, 45(1), 17–28.

Allen, P., & Narramore, R. (1985). Bedrock controls on stream channel enlargement with urbanization, North Central Texas. Water Resources Bulletin, 21(6), 1037–1048.

Anderson, D.G. (1970). Effects of urban development on floods in Northern Virginia (USGS Water Supply Paper 2001-C). Washington, DC: U.S. Geological Survey.

Bannerman, R., Owens, D., Dodds, R., & Hornewer, N. (1993). Sources of pollutants in Wisconsin stormwater. Water Science and Technology, 28(3–5): 241–259.

Black & Veatch Corporation. (1994). Longwell branch restoration—Feasibility study (Vol. 1). Westminster, MD: Carroll County, Maryland, Office of Environmental Services.

Booth, D. (1990). Stream channel incision following drainage basin urbanization. Water Resources Bulletin, 26(3), 407–417.

Booth, D., & Jackson, C. (1997). Urbanization of aquatic systems: Degradation thresholds, stormwater detection, and the limits of mitigation. Journal of the American Water Resources Association, 33(5), 1077–1090.

Boward, D., Kazyak, P., Stranko, S., Hurd, M., & Prochaska, T. (1999). From the mountains to the sea: The state of Maryland's freshwater streams (EPA 903-R-99-023). Annapolis: Maryland Department of Natural Resources.

Caraco, D. (2000a). The dynamics of urban stream channel enlargement. Watershed Protection Techniques, 3(3), 729–734.

Caraco, D. (2000b). Dynamics of urban stream channel enlargement. In T. Schueler & H. Holland (Eds.), The Practice of Watershed Protection. Ellicott City, MD: Center for Watershed Protection.

Caraco, D. (2000c). Stormwater strategies for arid and semi-arid watersheds. Watershed Protection Techniques, 3(3), 695–706.

Center for Watershed Protection. (1998). Better site design: A handbook for changing development rules in your community. Ellicott City, MD: Center for Watershed Protection.

Center for Watershed Protection. (1999). Why watersheds? An introduction to the whys and hows of water resource protection. Ellicott City, MD: Center for Watershed Protection.

Center for Watershed Protection (CWP). (2003). Impacts of impervious cover on aquatic systems (Watershed Protection Research Monograph No. 1). Ellicott City, MD: Center for Watershed Protection.

Center for Watershed Protection, (CWP); Macris, Hendricks, and Glascock, Inc.; and Environmental Systems Analysis, Inc. (2001). Watts Branch watershed study and management plan final report. Prepared for the City of Rockville, Maryland. Ellicott City, MD: Center for Watershed Protection.

Chin, N. (1996). Watershed urbanization effects on palustrine wetlands: A study of the hydrologic, vegetative, and amphibian community response over eight years.Master of Science thesis, Department of Civil Engineering, University of Washington, Seattle.

Crunkilton, R., Kleist, J., Ramcheck, J., DeVita, W., & Villeneuve, D. (1997). Assessment of the response of aquatic organisms to long-term in-situ exposures of urban runoff. In L.A. Roesner (Ed.), Effects of watershed development and management on aquatic ecosystems. Proceedings of the ASCE Discussion in Snowbird, Utah, August 4–9, 1996. New York: American Society of Civil Engineers.

Delzer, G.C., Zogorski, J.S., Lopes, T.J., & Bosshart, R.L. (1996). Occurrence of the gasoline oxygenate MTBE and BTEX compounds in urban stormwater in the United States, 1991–95 (U.S. Geological Survey Water-Resources Investigations Report 96-4145). Washington, DC: USGS.

Ehrenfeld, J.G. (2000). Evaluating wetlands within an urban context. Urban Ecosystems, 4, 69–85.

Ferrari, M., Altor, S., Blomquist, J., & Dysart, J. (1997). Pesticides in the surface water of the Mid-Atlantic region (U.S. Geological Survey Water-Resources Investigations Report 97-4280). Washington, DC: USGS.

Frederick County, Maryland, Division of Planning. (2006). New market region plan board of commissioners zoning. Frederick, MD: Frederick County Division of Planning/Frederick County GIS (Graphic Information System). http://www.co.frederick.md.us/Planning/CompPlan/NewMarket/BOCC-PR-ZON-NM.pdf

Galli, J. (1990). Thermal impacts associated with urbanization and stormwater management best management practices. Washington, DC: Metropolitan Washington Council of Governments, Maryland Department of the Environment.

Galli, J. (1994). Personal communication. Department of Environmental Programs, Metropolitan Washington Council of Governments, Washington, DC.

Hammer, T. (1972). Stream channel enlargement due to urbanization. Water Resources Research, 8(6), 1530–1540.

Hicks, A., & Larson, J. (1997). The impacts of urban stormwater runoff on freshwater wetlands and the role of aquatic invertebrate bioassessment. Amherst: University of Massachusetts, The Environment Institute.

Horner, R., Booth, D., Azous, A., & May, C. (1997). Watershed determinants of ecosystem functioning. In L.A. Roesner (Ed.), Effects of watershed development and management on aquatic ecosystems. Proceedings of the ASCE Discussion in Snowbird, Utah, August 4–9, 1996. New York: American Society of Civil Engineers.

Keller, A., & Zam, S. (1991). The acute toxicity of selected metals to the freshwater mussel, Anodonta imbecillis. Environmental Toxicology and Chemistry, 10, 539–546.

Klein, R. (1979). Urbanization and stream quality impairment. Water Resources Bulletin, 15(4), 948–963.

Legal Information Institute. (2006). Land use law: An overview. Ithaca, NY: Cornell Law School, Legal Information Institute. Retrieved July 28, 2006, from http://www.law.cornell.edu/wex/index.php/Land_use

Leopold, L.B., Wolman, M.G., & Miller, J.P. (1964). Fluvial processes in geomorphology. San Francisco: Freeman.

Limburg, K., & Schmidt, R. (1990). Patterns of fish spawning in Hudson River Tributaries—Response to an urban gradient? Ecology, 71(4), 1238–1245.

Lopes, T.J., & Dionne, S.G. (1998). A review of semivolatile and volatile organic compounds in highway runoff and urban stormwater (USGS Open File Report 98-409). Rapid City, SD: U.S. Geological Survey.

Luchetti, G., & Feurstenburg, R. (1993). Relative fish use in urban and non-urban streams. Proceedings of the Discussion on Wild Salmon. Vancouver, British Columbia, Canada.

Maryland-National Capital Park and Planning Commission (MNCPPC). (2000). Stream condition cumulative impact models for the Potomac subregion. Annapolis, MD: Environmental Resources Management Group.

Masterson, J., & Bannerman, R. (1994). Impact of stormwater runoff on urban streams in Milwaukee County, Wisconsin. Madison: Wisconsin Department of Natural Resources.

Maxted, J., & Shaver, E. (1997). The use of retention basins to mitigate stormwater impacts on aquatic life. In L.A. Rosener (Ed.), Effects of watershed development and management on aquatic ecosystems (pp. 494–512). New York: American Society of Civil Engineers.

Mellis, S., & Schuster, D. (2001). Home-a-syst environmental risk assessment guide—Fact sheet 2. Columbia, MO: University of Missouri, Columbia, Outreach and Extension.

Metropolitan Washington Council of Governments (MWCOG). (1992). Watershed restoration sourcebook. Washington, DC: MWCOG, Department of Environmental Programs.

Moring, J., & Rose, D. (1997). Occurrence and concentrations of polycyclic aromatic hydrocarbon in semipermeable membrane devices and clams in three urban streams of the Dallas-Fort Worth Metropolitan Area, Texas. Chemosphere 34(3), 551–566.

Morisawa, M., & LaFlure, E. (1979). Hydraulic geometry, stream equalization and urbanization. In Adjustments of the fluvial system. Proceedings of the 10th Annual Geomorphology Symposia Series, Binghamton, New York, September 21–22, 1979. Dubuque, IA: Kendall/Hunt.

Morse, C. (2001). The response of first- and second-order streams to urban land-use in Maine, USA. Master's thesis, University of Maine, Orono, ME.

Natural Resources Conservation Service (NRCS). (2003). 2003 national resources inventory: Land use. Available at http://www.nrcs.usda.gov/TECHNICAL/land/nri03/nri03landuse-mrb.html

Ohrel, R. (1995). Rating deicer agents—Road salts stand firm. Watershed Protection Techniques, 1(4), 217–220.

Pizzuto, J., Hession, W., & McBride, M. (2000). Comparing gravel-bed rivers in paired urban and rural catchments of Southeastern Pennsylvania. Geology, 28(1), 79–82.

Richardson, D. (2006). Parking lot sealants: On the trail of urban PAHs. Stormwater, 7(3), 40–46.

Robbins, P., & Birkenholtz, T. (2003). Turfgrass revolution: Measuring the expansion of American lawn. Land Use Policy, 20, 181–194.

Schueler, T. (1987). Controlling urban runoff: A practical manual for planning and designing urban best management practices. Washington, DC: Metropolitan Washington Council of Governments.

Schueler, T. (1994). The importance of imperviousness. Watershed Protection Techniques, 2(4), 100–111.

Schueler, T. (1995). Site planning for urban stream protection. Prepared for the Metropolitan Washington Council of Governments (MWCOG). Ellicott City, MD: Center for Watershed Protection.

Schueler, T. (1999). Microbes and urban watersheds. Watershed Protection Techniques, 3(1), 551–596.

Schueler, T. (2000). The compaction of urban soils. (Article 36.) In H. Holland & T. Schueler (Eds.), The Practice of Watershed Protection (pp. 210–214). Ellicott City, MD: Center for Watershed Protection.

Schueler, T. (2004). An integrated framework to restore small urban watersheds (Urban Subwatershed Restoration Series, Manual 1). Ellicott City, MD: Center for Watershed Protection.

Schueler, T., & Shepp, D. (1993). The quantity of trapped sediments in pool water within oil grit separators in suburban Maryland. Washington, DC: Metropolitan Washington Council of Governments (MWCOG).

Smith, B., & Darr, D. (2004). U.S. forest resource facts and historical trends. Washington, DC: U.S. Department of Agriculture.

Steuer, J., Selbig, W., Hornewer, N., & Prety, J. (1997). Sources of contamination in an urban basin in Marquette, Michigan, and an analysis of concentrations, loads, and data quality (USGS Water-Resources Investigations Report 97-4242). Washington, DC: U.S. Geological Survey.

Steward, C. (1983). Salmonid populations in an urban environment—Kelsey Creek, Washington. Master's thesis, University of Washington.

Stranko, S., & Rodney, W. (2001). Habitat quality and biological integrity assessment of freshwater streams in the Saint Mary's River watershed (Report CBWP-MANTA-EA-01-2). Annapolis: Maryland Department of Natural Resources.

U.S. Environmental Protection Agency (EPA). (1998). The quality of our nation's waters: 1996 (EPA-841-S-97-001). Washington, DC: U.S. EPA, Office of Water.

U.S. Environmental Protection Agency (EPA). (2000). National water quality inventory, 2000 Report. Washington, DC: U.S. EPA. Available at http://www.epa.gov/305b/2000report/

U.S. Environmental Protection Agency (EPA). (2006). Aquatic biodiversity: Freshwater ecosystems. Retrieved November 13, 2006, from http://www.epa.gov/bioindicators/aquatic/freshwater.html

U.S. Geological Survey (USGS). (1998). Pesticides in surface waters of the Santee River Basin and coastal drainages, North and South Carolina (USGS Fact Sheet FS-007-98). Washington, DC: USGS.

U.S. Geological Survey (USGS). (1999a). Pesticides and bacteria in an urban stream—Gills Creek, Columbia, South Carolina (USGS Fact Sheet FS-131-98). Washington, DC: USGS.

U.S. Geological Survey (USGS). (1999b). Pesticides detected in urban streams during rainstorms and relations to retail sales of pesticides in King County, Washington(USGS Fact Sheet 097-99). Washington, DC: USGS.

U.S. Geological Survey (USGS). (2001a). Selected findings and current perspectives on urban and agricultural water quality (USGS Fact Sheet FS-047-01). Washington, DC: USGS, National Water-Quality Assessment Program.

U.S. Geological Survey (USGS). (2001b). The quality of our nation’s waters: Nutrients and pesticides (USGS Fact Sheet FS-047-01). Washington, DC: USGS.

U.S. Geological Survey (USGS). (2006). Water resources of the United States: National water-quality assessment glossary. Retrieved November 16, 2006, from http://water.usgs.gov/nawqa/glos.html

Velinsky, D., & Cummins, J. (1994). Distribution of chemical contaminants in wild fish species in the Washington, D.C., area (Interstate Commission on the Potomac River Basin (ICPRB) Report No. 94-1). Rockville, MD: ICPRB.

Waschbusch, R., Selbig, W., & Bannerman, R. (2000). Sources of phosphorus in stormwater and street dirt from two urban residential basins in Madison, Wisconsin, 1994–1995. National Discussion on Tools for Urban Water Resource Management and Protection, February 2000, 15–55. Washington, DC: U.S. Environmental Protection Agency.

Watters, G. (1996). Reasons for mussel decline and threats to continued existence. Philadelphia: Academy of Natural Sciences. Available at http://coa.acnatsci.org/conchnet/uniowhat.html

Williams, J., Warren, M. Jr., Cummings, K., Harris, J., & Neves, R. (1993). Conservation status of freshwater mussels of the United States and Canada. Fisheries 18(9), 6–22.

Woodward-Clyde Consultants. (1992). Source identification and control report (Prepared for the Santa Clara Valley Nonpoint Source Control Program). Oakland, CA: Woodward-Clyde.

Wright, T. (2006). Direct and indirect impacts of land development on wetland quality (Wetlands and Watersheds Series). Ellicott City, MD: Center for Watershed Protection.

Wright, T., Swann, C., Cappiella, K., & Schueler, T. (2004). Unified subwatershed and site reconnaissance: A user's manual (Urban Subwatershed Restoration Series, Manual 11). Ellicott City, MD: Center for Watershed Protection.