Watershed Protection and Management
Lacevero
Module2-UnderstandingtheScienceofWatersheds.docxModule 2: Understanding the Science of Watersheds
Topics
III. Water Quality
Watershed science studies the relationship between land-use activities and their effect on aquatic ecosystems. Because this relationship is shaped primarily by water, hydrology is the foundation of watershed science. Watershed management is the application of watershed science to determine the most effective way to manage land and water resources in a watershed.
One of the most important objectives of watershed management is to determine how to deal with stormwater runoff—rainfall that collects on our rooftops, parking lots, and roadways during a storm event. For 2,000 years, engineers have worked to quickly move stormwater runoff away from developed areas to reduce flooding and ensure public safety. This is done primarily by putting runoff into pipes or creating concrete channels that efficiently move runoff downstream.
In recent years, scientists have begun to realize that this approach has detrimental effects on downstream receiving waters and also affects the natural hydrologic regime, water quality, biology, and the physical structure of streams. Today, most stormwater management approaches focus on reducing the volume of runoff from development sites and improving the quality of the runoff. To predict runoff's impacts on the watershed with future development, an understanding of how much runoff a given storm generates within an individual site or drainage area is needed.
In this lesson, we will cover the basics of watershed hydrology to help you understand how runoff is generated in a watershed. You will learn how precipitation characteristics influence runoff, what factors affect how much rainfall is converted to runoff, and how to predict runoff volume, identify different types of flow, and record the changes in streamflow over time. All of these are key to predicting the effect of runoff on the watershed.
We will discuss the impacts of runoff on streams in detail in module 3. You will learn more about stormwater management in module 4.
Precipitation
Precipitation is an important part of the hydrologic cycle and is of interest in watershed management because of its capacity to produce stormwater runoff. Too much rainfall can cause flooding, and too little can cause drought. Rainfall can also carry pollutants such as acid rain, which may contaminate our surface and ground waters.
Precipitation can be described by its intensity (how hard), duration (how long), frequency (how often), and depth (how much). The average rainfall characteristics vary from region to region as well. For example, the city of Portland, Oregon, which is located in the rainy Pacific Northwest, gets an average of 37.07 inches of rainfall per year. Some people may be surprised to hear that Portland gets about the same annual rainfall as Maryland (where UMUC is located), because it seems like it is always raining in Portland. The difference is in the frequency, intensity, and duration of rainfall. Compare the data provided in table 2.1 for rainfall in Portland and Baltimore, Maryland.
Table 2.1 Rainfall Characteristics for Baltimore, Maryland, and Portland, Oregon
|
Rainfall Characteristic |
Baltimore |
Portland |
|
Average annual precipitation (in inches) |
41.94 |
37.07 |
|
Average number of days per year with measurable precipitation (0.01 inch or more) |
115 |
153 |
Source: NOAA Comparative Climatic Data (2006)
As you can see, total annual rainfall is similar, but storms occur more frequently in Portland. This frequency, however, decreases sharply as rainfall depth increases. For instance, in Portland, the average number of days per year with rainfall greater than 0.5 inch is only 20, and the average number of days per year with rainfall greater than one inch is only 4.5.
Scientists have analyzed differences in rainfall patterns to define rainfall depths that have a statistical probability of happening over a given return period, such as 100 years (e.g., the "100-year storm"). This does not mean that the 100-year storm will occur only once every 100 years. It is more accurate to say that 100-year stormactually refers to a rainfall event that has a one-percent chance of occurring in any given year. 100-year storms vary in size, depending on local rainfall averages. To determine the rainfall depths associated with a specific recurrence interval, we use a statistical method called a frequency analysis to evaluate at least 10 years of rainfall data from a specific city or region.
Engineered stormwater treatment practices are designed to detain and treat runoff from storms with a specified return period because the storm characteristics directly influence the resultant runoff patterns. These storms are called design storms and typically include the 1-, 2-, 5-, 10-, 25-, 50-, and 100-year storms. Historically, most stormwater treatment practices have been designed to reduce flooding from the larger storm events (such as 100-, 50-, and 10-year storms). More recently, stormwater managers are designing practices to also improve water quality by focusing on the smaller events, particularly the 1-year storm. To give you an idea of how much rain is associated with a typical design storm, examples are provided in table 2.2 for Baltimore County, Maryland, and Portland, Oregon.
Table 2.2 Estimated Rainfall Depths for Various Design Storms
|
Design Storm |
Probability of Occurrence in a Given Year |
Estimated Rainfall Depth (in inches) over 24-Hour Duration |
|
|
|
|
Baltimore County |
Portland |
|
2-year storm |
50% |
3.2 |
2.4 |
|
10-year storm |
10% |
5.1 |
3.4 |
|
100-year storm |
1% |
7.1 |
4.4 |
Sources: CWP and MDE (2000), City of Portland Environmental Services (2004)
As you can see from table 2.2, a rainfall depth of 3.2 inches has a 50-percent probability of occurring over a 24-hour period in a given year in Baltimore County; but in Portland, a similar rainfall depth of 3.4 inches has only a 10 percent probability of occurring over a 24-hour period in a given year. This is due in part to differences in rainfall intensity. If you compare two storms of the same duration, the one with greater intensity will produce a greater rainfall depth. Some examples of duration and intensity for different design storms are presented in table 2.3 for Baltimore.
Table 2.3 Rainfall Intensity Estimates for Various Design Storms and Durations in Baltimore, Maryland
|
Design Storm |
Rainfall Intensity Estimates (in inches per hour) for Three Storm Durations |
||
|
|
10 Minutes |
30 Minutes |
60 Minutes |
|
2-year storm |
3.64 |
2.10 |
1.32 |
|
10-year storm |
5.20 |
3.18 |
2.07 |
|
100-year storm |
6.90 |
4.45 |
3.06 |
Source: Bonnin et al. (2004)
As you can see from the data in table 2.3, intense rainfall typically does not last very long. In general, higher intensities and longer durations yield higher rainfall totals. For example, a 2-year storm that lasts for one hour will produce more than twice as much rainfall as a 2-year storm that lasts for only 10 minutes. Similarly, a storm that lasts for 30 minutes and has an intensity of 4.45 inches per hour will produce twice as much rainfall as a storm of the same duration and an intensity of 2.1 inches per hour. To calculate the rainfall depths associated with the storms in table 2.3, use the following equation:
Rainfall Depth Calculation
P = i * d
where: P = rainfall depth in inches i = rainfall intensity in inches per hour d = duration of storm in hours
Rainfall intensity influences both the size and terminal velocity of raindrops. The large raindrops of an intense summer thunderstorm are about 5 millimeters in diameter and fall at a speed of more than 20 miles per hour (Envirocast, 2003a). By contrast, during a light drizzle, raindrops are less than one millimeter in diameter and fall out of the sky at a leisurely rate of about 1.5 miles per hour (Envirocast). Still, even tiny raindrops are quite powerful when they strike bare soil. Their force on impact can easily detach soil particles from the land and erode them. This is particularly important to consider during land clearing for silviculture (e.g., forestry), crop tillage, and at large construction sites. We will discuss soil erosion in more detail in module 3.
Stormwater Runoff
How much surface runoff is generated during an individual storm in a given drainage area, and how fast the runoff reaches the outlet, are two of the most important factors to consider in watershed management because of the effects of stormwater on streams and other aquatic systems. To manage these effects, we must first understand the processes that generate runoff. The following section summarizes the generation of surface runoff, runoff coefficients, calculating runoff volume, types of flow, and discharge. Module 3 provides a detailed discussion of the impacts of runoff on watersheds.
Surface Runoff Generation
When rain falls over the watershed, a portion of precipitation may never reach the ground because it is intercepted by vegetation and other surfaces. Forests in particular can greatly decrease the quantity of precipitation that ultimately reaches the ground through rainfall interception by the tree canopy and by releasing water into the atmosphere through evapotranspiration (figure 2.1).
Figure 2.1 Typical Pathways for Forest Rainfall
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Rainfall that actually reaches the ground may infiltrate the soil, or it may ultimately be converted into surface runoff. Both the soil's capacity to infiltrate water and the rainfall characteristics determine the amount of runoff generated. When the soil is saturated to the point at which it can no longer absorb rainfall, water begins to run off the land surface. The presence of trees and other vegetation can help promote infiltration of rainfall into the soil, but the presence of impervious surfaces such as asphalt and concrete greatly limits infiltration. Figure 2.2 illustrates how the soil infiltration capacity (measured by the infiltration rate) and rainfall intensity (also called rainfall rate) influence surface runoff generation. Because no vegetation is present in the example shown in figure 2.2, it is assumed that all of the rainfall actually reaches the ground.
Figure 2.2 Soil Infiltration and Runoff
Directions: Hold your mouse over each image for details.
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Runoff Coefficient, Rv
The runoff coefficient (Rv) is a measure of how much rainfall volume is converted into runoff. To manage runoff, we must be able to predict how much runoff will be generated during a typical rainfall. Schueler (1987) found that, in urban watersheds, impervious cover is a good predictor of the runoff coefficient, using the shortcut method shown below.
Shortcut Method of Calculating Runoff Coefficient
Rv = 0.05 + 0.009I
where: I = Impervious cover percentage in watershed
Impervious cover is defined as any surface that does not allow rainfall to penetrate the soil (e.g., pavement, rooftops). The shortcut method for predicting the runoff coefficient is recommended for watersheds that are predominately one cover type. The equation is derived from a study of the relationship between watershed imperviousness and mean runoff coefficient for 44 small urban watersheds nationwide (see figure 2.3). Watershed imperviousness is defined as the percentage of a watershed that consists of impervious surfaces, such as roads, parking lots, driveways, buildings, and sidewalks. Methods for measuring watershed imperviousness are summarized in Cappiella and Brown (2001). We'll learn more about impervious cover and its effects in module 3.
Figure 2.3 Relationship Between Watershed Imperviousness and Runoff Coefficient
Calculating Runoff Volume
The runoff coefficient is used to calculate the runoff volume from a drainage area for the purposes of stormwater management. To manage runoff, it is necessary to predict how much runoff will be generated during a typical rainfall for a specific drainage area. A simple equation for predicting runoff volume is shown below (Schueler, 1987):
Equation for Predicting Runoff Volume
R = P * Rv * A
where: R = runoff volume in cubic feet P = rainfall depth in feet Rv = runoff coefficient A = drainage area in square feet
Using the shortcut method to predict the runoff coefficient and the above equation to predict runoff volume, we can estimate and compare the runoff volume produced by a one-inch storm over a one-acre forest versus a parking lot. Table 2.4 compares the results.
Table 2.4 Comparison of Runoff Volume from a Forest and a Parking Lot
|
Parameter |
Forest |
Parking Lot |
|
Drainage area |
1 acre |
1 acre |
|
Land cover |
100% pervious |
100% impervious |
|
Rainfall depth |
1 inch |
1 inch |
|
Runoff coefficient |
0.05 |
0.95 |
|
Runoff volume |
181.50 cubic feet |
3,448.50 cubic feet |
As you can see from table 2.4, the volume of runoff produced from a parking lot is almost 20 times greater than the volume produced from a forest with the same drainage for the same-size storm. This is why stormwater management becomes so important when watersheds are developed.
Types of Flow
When runoff is generated in the watershed, it can take the form of sheetflow, shallow concentrated flow, open channel flow, or pipe flow. The type of flow is dependent on many factors, including the distance and surface over which the flow travels, the slope, development intensity, and average velocity.
Velocity is the rate of travel measured as distance-per-unit time. The velocity of runoff is measured in feet per second (fps) and is influenced by slope, surface roughness, and storm intensity and duration. To get an idea of how runoff velocity compares with some common real-world speeds, consider the data presented in table 2.5.
Table 2.5 Typical Velocities of Various Activities
|
Activity |
Velocity |
|
|
|
Miles Per Hour |
Feet Per Second |
|
Leisurely stroll |
1 |
1.5 |
|
Walk |
3 |
4.4 |
|
Fast run |
10 (6-minute miles) |
15 |
|
Driving on highway |
65 |
95 |
The type of flow is important because each type has a different effect on the watershed. For example, sheetflow is considered the most desirable form of flow from a watershed management standpoint because it generally has the least impact. A description of each type of flow is provided below.
Sheetflow
Surface runoff first begins as a thin sheetflow across the land surface. Sheetflow is very slow because of the roughness of the land surface. Typical velocities of sheetflow over different surfaces—forest, lawn, and pavement—are illustrated in table 2.6.
Table 2.6 Typical Velocities of Sheetflow over Forest, Lawn, and Pavement
|
Surface |
Typical Velocity (in feet per second) |
|
Forest |
0.05 |
|
Lawn |
0.17 |
|
Pavement |
0.50 |
Source: CWP (n.d.)
Shallow Concentrated Flow
In natural or unpaved areas, after about 150 feet, sheetflow begins to accumulate and is known as shallow concentrated flow (USDA SCS, 1986). Note that the water is not in a channel yet. If the land surface is paved, sheetflow concentrates in half this distance—about 75 feet (USDA SCS). The distance beyond which sheetflow concentrates will vary with slope and surface roughness. The average velocity of shallow concentrated flow is also a function of slope and surface roughness, as illustrated in table 2.7.
Table 2.7 Velocity as a Function of Slope
|
Slope (%) |
Unpaved Average Velocity (fps) |
Paved Average Velocity (fps) |
|
1 |
0.6 |
2.0 |
|
2 |
2.8 |
3.5 |
|
3 |
3.6 |
4.5 |
Source: USDA SCS (1986)
Open Channel Flow
When shallow concentrated flow attains sufficient velocity, it creates a defined flow channel with banks to confine the flow. Once in a channel, flow depths increase, roughness is reduced, and velocity increases. Slope is also very important in the velocity of open channel flow. Critical velocities exist where important things happen in a stream channel.
For example (CWP, n.d.):
· Suspended sediment is deposited below 2 fps.
· Erosion begins in grass channels at about 4.5 to 7 fps (depending on cover/slope).
· Streambed materials begin to roll:
· coarse sand 2 fps
· fine gravel 2.6 fps
· coarse gravel 5.4 fps
· stones 6 fps
· Upstream fish movement is thwarted at 2 to 10 fps (depending on size).
· Maximum reported flood velocity is 10 to 20 fps.
· Flow velocity in urban streams ranges from 0.5 to 20 fps in most storms.
Channel flow can be further categorized into stormflow and base flow. Stormflow is precipitation that reaches the channel over a short time frame via overland or underground routes. Base flow is precipitation that percolates to the groundwater and moves slowly through substrate before reaching the channel. Base flow sustains streamflow during periods of little or no precipitation. At any one time, streamflow may consist of water from one or both sources. We'll learn more about changes in stormflow and base flow characteristics that result from urbanization in module 3.
Pipe Flow
As mentioned before, for a long time the goal of stormwater management has been to move water off our streets and parking lots as quickly as possible. When watersheds are developed, impervious cover generates more stormwater runoff. An artificial drainage network of storm drains, inlets, and underground pipes is often constructed to move stormflows. Small channels are often piped and placed underground in an attempt to manage stormflows. Table 2.8 shows the typical velocity and drainage area associated with different pipe sizes. In general, smaller pipes drain smaller areas and have lower discharge and velocity, and the opposite is true for large pipes. As you can see from table 2.8, the velocity of pipe flow is significantly greater than velocities associated with any of the other types of flow. We'll learn more about how this increase in velocity affects streams in module 3.
Table 2.8 Characteristics of Various Pipe Sizes (for pipes flowing full, with one percent slope)
|
Pipe Diameter (in inches) |
Typical Area Drained (in acres) |
Average Velocity (in fps) |
|
6 |
0.1 to 1 |
4 |
|
12 |
1 to 2 |
6 |
|
24 |
2 to 5 |
10 |
|
36 |
5 to 25 |
12 |
|
48 |
25 to 100 |
14 |
|
60 |
100 to 200 |
18 |
Source: Kitchell and Schueler (2004)
Discharge
Discharge is the rate of flow at a given point in time. Dimensionally, it is a volume-per-unit time. An equation is provided below to calculate discharge, and figure 2.4 illustrates stream-channel discharge. You can see additional information by moving your mouse over the graphic.
Discharge Calculation
Q = AV
where: Q = discharge (cubic feet per second or cfs) A = cross-sectional area of channel (square feet) V = velocity (feet per second)
Figure 2.4 Stream-Channel Discharge
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
But what, really, is a cubic foot per second? To put this into perspective, consider the following:
· 1.55 cfs = 1 million gallons per day
· 1 cfs = 449 gallons per minute
· 1 cfs = 1 inch of runoff in one hour from a one-acre parking lot
Discharge is the "currency" of streams and increases with area, impervious cover, storm size, and "improved" conveyance, such as storm drainage pipes. The change in discharge over time in a stream is shown in a hydrograph. As illustrated in figure 2.5, a hydrograph consists of a rising limb, a peak, and a recession limb. The hydrograph shows how long it takes a stream to rise from base flow to peak discharge for a given storm and then to return to base flow conditions. As shown in the graphic, the discharge peak does not coincide with the peak rainfall because of the time it takes for runoff to form on the land surface and ultimately reach the stream. The difference between the peak rainfall and the peak discharge is called the lag time .
Figure 2.5 A Storm Hydrograph
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Figure 2.6 compares hydrographs for a stream before and after urbanization. You can see that the peak discharge is greater, and the lag time is much shorter for an urbanized watershed. We'll learn more about the impacts of urbanization on stream hydrology in module 3.
Figure 2.6 A Comparison of Hydrographs Before and After Urbanization
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Geomorphology is the study of landforms. In watershed science, the major landforms of interest are stream corridors. Stream corridors include the existing network of stream channels and the lands that immediately surround them. In this lesson, we will learn about the geomorphic processes that form streams. We will also discuss the type of stream discharge that governs the shape and form of stream channels and learn about the functions of floodplains. Finally, we will introduce you to specific features of the stream channel that define stream geomorphology.
Stream Geomorphic Processes
Streams are open channels that transmit flow all or part of the year. Streams are formed by flowing water under natural conditions over a period of time (hundreds or thousands of years). The geomorphic processes that form streams include erosion, sediment transport, and sediment deposition. Although these processes occur naturally, human activity can accelerate them.
Streams balance the erosion, sediment transport, and sediment deposition processes until they reach a state of equilibrium. In an equilibrium (i.e., stable) state, a stream's sediment load must be equal to its sediment transport capacity. In other words, the sediment inputs must equal the sediment outputs. When a stream is in a stable state, it essentially functions as a conduit for transporting sediment downstream. The sediment comes from the stream channel or is transported to the stream by overland flow or runoff over the land surface.
A stream's sediment load includes both sediment that is suspended in the water column (suspended sediment) and sediment that is rolled or otherwise moved along the streambed ( bedload ). Sediment load is typically measured in tons per year. When a stream's sediment load is greater than its capacity to transport sediment, the stream will deposit the excess sediment within its channel and floodplain. Streams that accumulate deposited sediment in this manner are called aggrading streams . So what causes a stream's sediment load to become greater than its transport capacity? Most often, it is accelerated erosion of sediment in the watershed from activities such as agriculture, mining, or land development, although extreme natural events may also be the cause.
When a stream's sediment load is less than its capacity to transport sediment, the stream will begin to erode away its channel bottom and/or banks. Streams that scour their channels in this manner are called degrading streams . The eroded sediment is then transported further downstream. A stream may begin to degrade due to construction of a dam or other structure upstream, which essentially 'starves' the stream of its normal sediment supply.
Bankfull Discharge
When a stream becomes unstable as described above, it adjusts its geometry in response to the changes in sediment load so as to re-establish its stable state. Although large, catastrophic events such as flooding are certainly important, it is generally acknowledged that the bankfull discharge, defined as discharge from storms with a recurrence interval of one and a half to two years, controls the shape and form of natural channels (Leopold, Wolman, & Miller, 1964; Anderson, 1970). Thus it is the smaller, more frequent storms that determine channel geometry—shape, dimensions, or slope. Figure 2.7 illustrates the elevation and width of the bankfull or channel-forming discharge in a stream channel. Just as the name indicates, the bankfull discharge represents flow that fills the stream channel to the top of its banks and begins to spill over onto the floodplain. You can see term definitions by moving your mouse over them.
Figure 2.7 Floodplain and Bankfull Discharge Measurements in a Stream Channel
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Bankfull elevation can be determined in the field using indicators such as deposits of silt, vegetation, or trash, or watermarks or silt stains on vegetation. You can determine the depth of the bankfull discharge in the field by measuring the height from the stream-channel bottom to the bankfull elevation. In a stable stream, bankfull depth is the same as the stream bank height (shown in figure 2.9).
Floodplains
Figure 2.7 also illustrates both the hydrologic and topographic floodplain of the stream. The hydrologic floodplain is defined by the bankfull width, and the topographic floodplain includes land on either side of the stream up to an elevation reached by a defined flood peak, usually the 100-year flood (FISRWG, 1998). The topographic floodplain may be inundated only during the most extreme events. It is important to remember that floodplain boundaries are not static because streams continually adjust their shape and form; and over time, the stream channel will migrate laterally across the stream valley floor, resulting in migration of the floodplain as well.
Floodplains are important for several reasons. During major storms, increased stream flow velocities can cause streams to erode their channels. When these larger flows overtop the banks onto the floodplain, the stream loses some of its power and its capacity to transport sediment is reduced, resulting in deposition of organic matter and sediment onto the floodplain. This depositional material enriches soils and promotes plant growth, often producing exceedingly fertile farmland.
Floodplains also provide temporary storage of floodwaters, reducing the risk of flooding farther downstream. In areas where the natural floodplain has been filled and developed, or where levees prevent floodwaters from reaching the floodplain, floodwaters have nowhere to go but downstream, and the risk of flooding is increased due to reduced local storage. In module 3, we'll discover how watershed changes caused by urbanization affect these functions of floodplains, bankfull discharge, andstream-channel geometry. The next section of this lesson discusses stream-channel geometry in more detail.
Stream-channel Geometry
Stream-channel geometry refers to the shape and dimensions of a stream channel. Stream-channel geometry is important because it tells you something about the processes that formed an individual stream and its current state (e.g., stable, aggrading, degrading). The latter is vital when restoring urban streams to a stable state. We will not go into detail on stream-channel design in this course; however, an important point to remember is that the design of stream-channel restoration projects must take into account any future land-use changes in the watershed. It is pointless to restore a stream to a stable state only to develop upstream and have the increased runoff cause the stream to degrade again.
It is important to recognize and measure lateral and longitudinal stream geometry features when evaluating streams in the field. Some of the most important features and measurements are shown in figures 2.8 and 2.9. Move your mouse over each feature in these graphics to see a definition. Figure 2.8 illustrates a stream that is fairly sinuous. Sinuosity is a measure of how much a channel meanders; it is measured as the ratio of the stream-channel length between two points on a channel and the straight-line distance between the same two points.
Figure 2.8 Plan View of Stream-Channel Features
Figure 2.9 Profile of a Stream Channel
Water quality refers to a measure of the chemical constituents of water, be it surface water, groundwater, drinking water, or urban runoff. It is important to protect and maintain the quality of all water sources because they are all interconnected through the hydrologic cycle and ultimately affect our drinking water supplies, swimming beaches, fishing holes, and other environmental conditions. Literally hundreds of contaminants may be found in our waters. Table 2.9 lists common pollutants and their sources. Water quality testing and monitoring can be extremely expensive, due in large part to the wide variety of possible pollutants present. Some important water quality terminology used in this module is defined below.
· A pollutant concentration is the amount of a particular chemical constituent that is measured in the water column, and is expressed in weight per volume of water (e.g., milligrams per liter [mg/L]).
· An event mean concentration (EMC) is the average pollutant concentration found in stormwater runoff. It, too, is expressed in mg/L or similar units.
· EMCs are used to calculate pollutant loads generated from urban, agricultural, and sometimes forested land uses. Pollutant loads are rates of export of a pollutant from a watershed or other defined area over a specified time period. They are typically expressed in pounds per acre per year.
These three measures are important from a watershed-management perspective because an important goal of most watershed plans is to reduce loads of one or more pollutants of concern. To do this, one must be able to first calculate the current pollutant load, and then estimate the load reduction expected when various management measures are applied. In this lesson, we will review basic water quality terminology and standards and discuss common urban pollutants and sources, as well as sources of data for pollutant concentrations in urban runoff. In module 4, we will review various methods of reducing pollutant loads in the watershed.
Table 2.9 Common Water Pollutants and Their Sources
|
Pollutant |
Primary Sources |
|
sediment |
construction sites, bank erosion, mining, agricultural runoff, urban runoff |
|
nutrients (nitrogen, phosphorus) |
fertilizer, organic wastes, detergents, and sewage in urban runoff; fertilizer in agricultural runoff; atmospheric deposition, bank erosion |
|
metals (lead, copper, zinc, iron, cadmium) |
urban runoff, atmospheric deposition, vehicles |
|
petroleum hydrocarbons (polycyclic aromatic hydrocarbons, oil, and grease) |
urban runoff from parking areas, gas stations, and highways; leaking storage tanks |
|
bacteria (fecal coliform, fecal strep, E. coli) |
urban runoff, runoff from livestock areas, leaking septic systems, illicit discharges, sanitary sewer overflows, combined sewer overflows, wildlife |
|
pathogens (cryptosporidium, giardia) |
urban runoff, runoff from livestock areas, leaking septic systems, illicit discharges, sanitary sewer overflows, combined sewer overflows, wildlife |
|
organic matter (biochemical oxygen demand, total organic carbon) |
urban runoff, agricultural runoff, bank erosion, forests |
|
pesticides |
urban runoff from lawns, golf courses, ball fields, and nurseries; agricultural runoff, pesticide storage areas |
|
chloride |
runoff from roads and parking lots in winter; salt storage areas, irrigation return flow |
Surface-Water Quality in the United States
In the United States, the quality of surface waters is monitored regularly, as mandated by the 1972 Clean Water Act (CWA). This national water quality–monitoring program is called the 305(b) program, referring to that section of the CWA. States monitor their surface waters periodically (every two years) and report the results to EPA authorities, who then compile a summary report. To determine if surface-water quality is being degraded, states must first set a baseline from which to measure. Therefore, the other component of this program involves establishing specific water quality standards for each water body, including both pollutant concentration thresholds and designated uses (e.g., contact recreation, fisheries). If monitoring results exceed the established water quality standards, the water body is considered impaired, or unable to support its designated uses. The EPA also tracks impaired waters at a national scale under the 303(d) impaired waters list. Once a waterway is classified as impaired, a Total Maximum Daily Load (TMDL) must be established to help determine the sources of pollution and a discharge allocation that will meet water quality standards.
An example of designated uses and related water quality standards is presented in table 2.10 for the state of Rhode Island. The criteria represent the mean concentration of fecal coliform bacteria that must not be exceeded for the class of water listed. As you can see, the criteria are increasingly stringent for waters designated for use as drinking water supply, shellfish harvesting, and primary contact recreation. The water quality criteria are not applicable to certain classes of water (e.g., Class C and Class SC) because it is presumed that the intended use of these waters (e.g., secondary contact recreation, such as boating) will not be affected by an exceedance of the water quality criteria.
Table 2.10 Rhode Island State Class-Specific Water Quality Criteria for Total Coliform
|
Class of Waters and Designated Uses |
Water Quality Criteria for Total Coliform Bacteria (MPN/100ml) |
|
|
Freshwater |
Class A: drinking water supply, primary and secondary contact recreation, fish and wildlife habitat, industrial and agricultural use, aesthetics |
100 |
|
|
Class B: primary and secondary contact recreation, fish and wildlife habitat, industrial and agricultural use, aesthetics |
1,000 |
|
|
Class C: secondary contact recreation, fish and wildlife habitat, industrial and agricultural use, aesthetics |
not applicable |
|
Seawater |
Class SA: shellfish harvesting for direct human consumption, primary and secondary contact recreation, fish and wildlife habitat, industrial and agricultural use, aesthetics |
70 |
|
|
Class SB: shellfish harvesting for controlled relay and depuration, primary and secondary contact recreation, fish and wildlife habitat, industrial and agricultural use, aesthetics |
700 |
|
|
Class SC: secondary contact recreation, fish and wildlife habitat, industrial and agricultural use, aesthetics |
not applicable |
Source: State of Rhode Island and Providence Plantations Department of Environmental Management (1997)
According to the latest 305(b) monitoring report from EPA (2000), the following percentages of U.S. waters assessed were found to be impaired for one or more uses:
· streams and rivers: 39 percent
· ponds, lakes, and reservoirs: 46 percent
· estuaries: 51 percent
· Great Lakes shoreline: 78 percent
Very few acres of wetland, groundwater, or ocean/marine resources are evaluated due to limitations on how to monitor or set standards for these resources. The top causes of impairment were found to be siltation, nutrients, bacteria, metals (primarily mercury), and oxygen-depleting substances. The most common source of pollution was runoff from urban and agricultural land (EPA, 2000). We will discuss these and other pollutants, and their sources and effects on watersheds, in module 3.
Point-Source Pollution
Pollutants in surface and ground water can come from point or nonpoint sources. Point sources are pollutant loads discharged at a specific point location, such as a pipe, outfall, or conveyance channel. Typical point sources include discharges from municipal wastewater treatment plants or industrial waste treatment facilities. Other point sources may include sanitary sewer overflows, combined sewer overflows, and certain illicit discharges.
Until 1987, the CWA regulated only point sources of pollutants by requiring a permit for all point discharges to surface waters. All industrial, municipal, and federal facilities that discharge wastewater directly to surface waters must obtain a National Pollutant Discharge Elimination System (NPDES) permit. These discharges typically include treated wastewater from sewage treatment plants, cooling water from industrial facilities, and flush water from quarries. Each permit is granted for a specified time period and sets limits on the amount of a pollutant that can be discharged.
Other point sources that are not regulated by permit include sewer overflows and illicit discharges. Sanitary sewer overflows (SSOs) occur when pipes are clogged or overloaded during rainstorms due to water infiltration into deteriorating sanitary lines. Combined sewer overflows (CSOs) also occur during rainstorms and periods of high water use because the combined sewer system collects and treats both stormwater runoff and sewage, and the capacity of the treatment plant may be exceeded during these high flows. Both types of sewer overflows can discharge untreated wastewater into basements, streets, or nearby streams.
Illicit discharges are accidental or intentional discharges other than stormwater into storm drain pipes. A variety of pollutants in local waterways have been found to derive from illicit discharges. An example of an illicit discharge is an illegal connection to the storm drain system through a floor drain in an auto repair shop, allowing oil, grease, and other contaminants to be washed down the drain and into local streams.
Nonpoint-Source Pollution
Nonpoint-source pollution does not have a single point of origin, but rather comes from dispersed sources, such as runoff from urban and agricultural land. Therefore, pollutant loads are directly related to land cover.
· Forest cover is the best use of land in a watershed in terms of reducing runoff of pollutants such as nutrients (Cappiella, Schueler, & Wright, 2005). Forests act as a sink for nutrients and lock them up in live and dead biomass, as well as soils. As a result, measured nutrient concentrations in forest runoff are quite low (Mostaghimi et al., 1994; USGS, 1999).
· Turf, on the other hand, generates much higher nutrient levels, according to source area monitoring of both fertilized and unfertilized lawns (Garn, 2002; Waschbusch, Selbig, & Bannerman, 2000; Steuer, Selbig, Hornewer, & Prety, 1997; Bannerman, Owens, Dodds, & Hornewer, 1993).
· Impervious cover produces intermediate nutrient concentrations that reflect the washoff of nutrients deposited from the atmosphere, lawn runoff, car exhaust, or household pets (CWP, 2003).
Figure 2.10 presents median nutrient concentrations in stormwater from these three types of land cover:
Figure 2.10 Median Nutrient Concentrations in Stormwater (mg/L)
Created by the authors based on Mostaghimi et al. (1994), USGS (1999), Garn (2002), Waschbusch et al. (2000), Steuer et al. (1997), and Bannerman et al. (1993)
Nutrient concentrations are only part of the story:
· Forests act as a sponge for rainfall and produce very little, if any, stormwater runoff. The forest canopy intercepts rainfall, and the remainder soaks into the forest floor. Forest monitoring has shown that less than 5 percent of rain falling on a forest is converted into runoff (Mostaghimi et al., 1994).
· Turf cover, on average, has a runoff coefficient twice as high as a forest, although the coefficient tends to vary considerably depending on the soil type, age, and soil compaction of the lawn (Legg, Bannerman, & Panuska, 1996; Pitt, 1987).
· As might be expected, nearly all of the rain that lands on impervious cover is converted into stormwater runoff (Schueler, 1987).
Figure 2.11 presents runoff coefficients for these three land cover types.
Figure 2.11 Runoff Coefficients for Different Land-Cover Types
Created by the authors based on Schueler (1987)
The product of runoff volume and pollutant concentration yields the annual nutrient load (Cappiella et al., 2005). Using the nutrient concentrations and runoff coefficients presented in figures 2.10 and 2.11, annual nutrient loading for forest, turf, and impervious cover were calculated using the Simple Method (Schueler, 1987) and an average annual rainfall of 40 inches. Figure 2.12 illustrates these results. Clearly, forests are the most desirable form of watershed cover when it comes to nutrient loading. For example, an acre of turf is calculated to produce 15 times more nutrients than an acre of forest cover. The difference is even more significant when forest cover is compared with impervious cover—more than 25 times more nitrogen and phosphorus are lost from impervious cover. The water quality benefits of maintaining or increasing forest cover can be impressive at the watershed scale.
Figure 2.12 Annual Nutrient Loading in Stormwater Runoff (pounds/acre/year)
Developed by the authors based on the data presented in figures 2.10 and 2.11, using the Simple Method (Schueler, 1987) and assuming annual rainfall of 40 inches
In 1987, Congress amended the Clean Water Act to regulate nonpoint sources in addition to point sources, based on the evidence that nonpoint sources were major contributors to pollution in U.S. waters. States are now required to assess and control nonpoint-source pollution, and NPDES permits are required for stormwater discharges from sources such as municipal separate storm sewer systems (MS4s) serving populations of a specified size, as well as several categories of industrial activity, such as construction sites.
Sources of Pollutant-Loading Data
As mentioned previously, estimating pollutant loads is important in watershed management to establish water quality impairments and improvements. A key piece of data needed to estimate pollutant loads is the event mean concentrations (EMCs) for pollutants of concern. Ideally, the data will be derived from a local or regional study. Because few such studies existed, however, the National Urban Runoff Program (NURP) conducted urban-runoff monitoring in the 1970s and 1980s to quantify pollutant loads at a national scale. More recent urban-runoff monitoring data are available through the National Stormwater Quality Database (NSQD), a creation of the University of Alabama and the Center for Watershed Protection. The NSQD is a compilation of data that regulated MS4 communities were required to collect as part of the NPDES Phase I permit application to quantify pollutant loads in their urban runoff. The NSQD provides a more recent and reliable update to the NURP data, provides EMCs for more specific land-use categories than NURP, and is the largest urban stormwater database ever compiled.
The most recent version of the NSQD contains data from 3,770 separate storm events from 66 agencies and municipalities across 17 states (Pitt et al., 2004). Because the project originally focused on the Chesapeake Bay region, most of the data are concentrated there; however, the database has good representation from other regions. Only 11 states are not well represented due to a lack of NPDES communities (Pitt et al.). The majority of data were from residential, industrial, and commercial land uses. The database includes information for about 125 different stormwater quality constituents, although it is mostly populated with data from 35 commonly analyzed pollutants (e.g., sediment, nutrients, metals, bacteria) (Pitt et al.). Table 2.11 presents the EMCs for selected pollutants in the NSQD for residential, commercial, industrial, and roadway land uses.
Table 2.11 EMCs for Selected Pollutants in the NSQD
|
Parameter |
Residential |
Commercial |
Industrial |
Freeways |
|
total suspended solids (mg/l) |
48 |
43 |
77 |
99 |
|
fecal coliform (mpn/100 ml) |
7,750 |
4,500 |
2,500 |
1,700 |
|
NH3 (mg/l) |
0.31 |
0.5 |
0.5 |
1.07 |
|
nitrate plus nitrate (mg/l) |
0.6 |
0.6 |
0.7 |
0.3 |
|
total phosphorus (mg/l) |
0.3 |
0.22 |
0.26 |
0.25 |
|
total lead (ug/l) |
12 |
18 |
25 |
25 |
|
total zinc (ug/l) |
73 |
150 |
210 |
200 |
Source: Pitt et al. (2004)
You can use data from the NSQD to calculate pollutant loads for your watershed. As you can see in table 2.11, pollutant loads will vary depending on the mix of different land-use types present in your watershed. Some land-use types have higher concentrations of certain pollutants than others. For example, freeways are a major source of metals such as zinc and lead, and residential areas have the highest concentrations of fecal coliform bacteria.
Watershed biology refers to the characteristics, biodiversity in particular, of living organisms within a watershed. Biodiversity is the variety of all life forms on Earth, such as plants, animals, and microorganisms. It refers to species diversity (i.e., the number of different species), genetic diversity (i.e., variation within species), and ecosystem diversity.
In watershed biology, biodiversity is typically measured in terms of the abundance of species within a given population, and the species richness within a given community:
· A population is a group of individuals of the same species living in a particular area.
· A community consists of populations of different species living in a particular area.
· Species abundance is the number of individuals within a population.
· Species richness is the number of different species found within a specific community.
The distribution of species is also a factor of interest to biodiversity.
So why is diversity so important? One reason is that the community as a whole is better able to withstand disturbance with a variety of species than without. For example, species-specific diseases such as Dutch Elm disease can wipe out an entire community if elms are the only species present; however, if a diverse community is present, the die-off of one species will not eliminate the entire community. Loss of diversity can lead to the loss of ecosystem viability.
This lesson focuses on identifying important habitats at two different scales: the scale of the individual plant community, and the watershed scale. We describe the characteristics of individual plant communities that help to promote biodiversity. We also describe some habitat types that are important at the watershed scale for wildlife habitat, biodiversity, and other watershed benefits. Both scales are important in watershed management when identifying important habitats for conservation. This lesson also reviews the use of biological indicators to evaluate watershed condition.
Habitat for Wildlife
The characteristics of individual plant communities affect which wildlife species live there. In fact, many animal species are associated with specific plant communities that contain the food, shelter, and breeding grounds to which they have adapted. In general, plant communities that have both vertical and horizontal complexitysupport a more diverse population of wildlife species than relatively homogenous communities. Vertical complexity evaluates the variety of vertical vegetative layers in a community. In a forest, this is the mix of canopy, midstory, understory, and groundcover vegetation. Check your understanding of these terms in figure 2.13, which illustrates vertical complexity in a forest. The more layers of vegetation present in a community, the greater the diversity of species it can support.
Figure 2.13 Vertical Complexity in a Forest
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Horizontal complexity refers to the mix of different habitat types over a horizontal gradient. Riparian wetlands and floodplains provide a great example of horizontal complexity. Because they are transitional areas between uplands and water bodies, they often have different zones that get increasingly wet as you approach the stream or adjacent water body. As a result, the vegetation types in each zone also vary and the wetland as a whole can support a diverse mix of wildlife species. For this reason, wetlands are some of the most biologically diverse habitats in the world.
A stream system with high horizontal complexity will have a mix of different habitat types, such as substrate, aquatic vegetation, snags, pools, riffles, submerged logs, undercut banks, and rootmats. Fish, aquatic insects, and other organisms use these areas as habitat, so a greater variety will support a greater diversity of species.
Biodiversity within individual habitats is perhaps most strongly influenced by disturbances from nearby land-use activities, such as road construction or deforestation. These disturbances affect the condition of the habitat, rendering it unable to support a large or diverse wildlife population. We'll learn more about the impacts of urbanization on plant and wildlife communities in module 3.
Habitat for Watersheds
Plant communities that may be found in an undisturbed watershed most commonly include forests, wetlands, and grasslands. The distribution and characteristics of these different vegetative communities in a watershed are determined by factors such as climate, topography, soil characteristics, and water availability. Humans frequently convert these community types to non-native communities, such as cropland or turf, for the purposes of food production, recreational use, or aesthetics. The types, distribution, and characteristics of these plant communities in a watershed can directly affect watershed health. Therefore, watershed management strategies often focus on identifying important natural areas across the watershed to conserve because they have some benefit to the watershed. These "conservation areas" include mature, contiguous forest tracts, riparian corridors, headwater streams, wetlands, and habitat for rare, threatened, and endangered (RTE) species, which are defined and discussed later in this lesson.
Contiguous Forest
Forested land that is without significant breaks such as roads, power lines, or other clearings is defined as contiguous (CWP, 2002a). Contiguous forest permits the movement of forest animals in search of food, habitat, and breeding purposes. Contiguous forest also serves as a buffer between predators associated with the forest edge and the species in the interior.
The larger and rounder a tract of contiguous forest, the greater the amount of interior forest that is created (CWP, 2002a). Interior forest is commonly defined as forest that is at least 100 meters (330 feet) from the forest edge (Wilcove, 1985) and is important for many species of birds, wildlife, and plants (Wenger, 1999). Interior forest has very different characteristics from forest edges, and these differences become more pronounced with increased distance between the interior and the edge. Edge habitat occurs at the boundaries between the forest and adjacent land use (e.g., a field) and is more diverse, while interior habitat is generally more sheltered and homogeneous. Figure 2.14 illustrates edge and interior habitat in a forest.
Figure 2.14 Edge and Interior Habitat in a Forest
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Although both interior and edge habitats are valuable, human activities tend to greatly increase the amount of edge habitat; therefore, protection and restoration of interior habitats (i.e., large, contiguous forest tracts) is often the focus of watershed management. We'll learn more about how fragmentation of habitat contributes to the "edge effect" in urban watersheds in module 3.
Mature, contiguous forests are often prioritized for conservation over young forest tracts because mature forests provide habitat for many animal species that young forests cannot (CWP, 2002b). These forests contain mature hardwoods that produce nuts and acorns that serve as food for mammals, and large snags (standing dead trees) that woodpeckers and other cavity-nesting birds require for nesting habitat (CWP). Insects thrive on the decaying logs found here and themselves provide food for migrating songbirds and other resident species. Declines in populations of wildlife that depend on forest interior, such as songbirds, can be partially attributed to loss of mature forest stands (CWP).
Riparian Corridors
Forests are also valuable to wildlife as corridors that serve as travel-ways between patches of habitat. The degree to which different habitat patches are linked to one another through corridors is called connectivity. Connectivity is especially important in a riparian forest community. Figure 2.15 shows one riparian forest community with high connectivity and one without.
Figure 2.15 Riparian Forest Community with (A) High Connectivity, and (B) Fragmentation
Source: Stream Corridor Restoration: Principles, Processes, and Practices (October 1998), by the Federal Interagency Stream Restoration Working Group (FISRWG)
Riparian forests, found primarily in humid climates, provide multiple benefits for aquatic life. Trees provide leaf litter, which is an important source of energy to streams because it is the basis for aquatic community food webs. Quite simply, streams depend on autumn leaves to supply the energy needed to support stream life throughout the rest of the year (Envirocast, 2003b). A typical acre of mature forest will drop between two and three tons of leaves, twigs, and branches every fall (Envirocast). When these leaves blow into a stream, they form "packs" that are gradually broken down by fungi and bacteria, depending on temperature and current velocity (Envirocast). The fungi are a major food source for insects like caddisflies and stoneflies, which in turn are a food source for small fish and other aquatic life (Envirocast). Thus, fallen leaves are the base of the food chain in small streams and provide as much as 75 percent of the energy used in the food chain (Envirocast). Indeed, the pulse of fallen leaves in autumn triggers the biggest phase of growth and activity in small streams.
Trees also provide large woody debris, which creates habitat for fish, macroinvertebrates, amphibians, and reptiles. Large woody debris is defined as being a minimum of three feet long and six inches in diameter and is touching the water surface (Schueler & Brown, 2004). Streams with large woody debris can retain sediment, nutrients, and carbon more effectively and have good habitat structure. In addition to providing leaf litter and large woody debris, riparian forests shade the stream surface, regulating stream temperature for the organisms that live there, and provide habitat for terrestrial and amphibian wildlife.
Trees also improve soil and water quality through uptake of soil nutrients (primarily nitrogen), filtration of sediment and associated pollutants from runoff, and removal of pollutants commonly found in runoff. Riparian forests essentially serve as a buffer between a stream and adjacent land uses that can remove pollutants from runoff before they reach the stream.
Habitat for RTE Species
Rare, threatened, and endangered (RTE) species are those that are found in such limited numbers that they receive special consideration or protection by federal and/or state regulations. Threatened and endangered species are protected by the Federal Endangered Species Act of 1973, which is administered by the U.S. Fish and Wildlife Service and the National Marine Fisheries Service. Endangered means that a species is in danger of extinction throughout all or a significant portion of its range (USFWS, 2006). Threatened means that a species is likely to become endangered within the foreseeable future throughout all or a significant portion of its range (USFWS, 2006). Some states also designate species as rare, meaning that they exist in such low numbers or isolated populations that they may become threatened or endangered if their environment is disturbed.
RTE status can be defined on a global, national, regional, state, or local scale. Information about RTE species can be obtained from federal or state agencies (such as state natural heritage programs), or from a national source like The Nature Conservancy. To protect RTE species, their exact locations are not usually publicly available. You can usually get a listing of species found in your area, however, or a map showing the general boundaries of the environment in which the species are found.
Habitat is the key factor when locating and protecting RTE species. Critical habitat includes geographic areas that contain the physical or biological features essential to the conservation of a species and that may need special management or protection (USFWS, 2006). It is important to protect potential critical habitats as well as those in which RTE species have been confirmed. In addition, habitat in which one RTE species is found may also contain several other RTE species that have not yet been inventoried. Conservation of these sites is critical to conserving biodiversity in the watershed.
Wetlands
A wetland is an area that is regularly saturated by surface water or groundwater and is characterized by a prevalence of vegetation that is adapted for life in saturated soil conditions (EPA, 1994). Wetlands improve water quality by removing pollutants, minimize flood damage by slowing and storing floodwaters, and protect shorelines from erosion by absorbing storm surges. Wetlands can provide needed groundwater recharge and supply habitat for birds and wildlife, as well as recreational and educational open space for watershed residents. Different wetland types provide different functions, depending on their type, location in the watershed, and other factors like condition. Wright et al. (2006) provide a more detailed review of the benefits that wetlands provide to watersheds.
Headwater Streams
The location of habitats in the watershed is also an important factor when identifying conservation areas. Headwater streams and their associated floodplains and wetlands are crucial in watershed management because they dominate the landscape through their sheer number and cumulative length. Headwater streams provide many ecosystem services, including flood control, groundwater recharge, sediment filtration, nutrient cycling, organic matter input, and a diversity of wildlife habitat. Many species depend on small headwater streams at some point in their life cycle (e.g., trout and salmon use them for spawning). Headwater areas are usually linked to the larger stream network through groundwater, so their importance is not always immediately visible.
What happens in the local landscape is directly translated to headwater streams, and major receiving waters are, in turn, affected (CWP, 2000). As urbanization increases, streams handle increasing amounts of runoff, which degrades headwater streams as well as major tributaries (CWP). Conserving headwater stream habitats is important in watershed management because (1) these areas are exceptionally vulnerable to watershed changes, and (2) their cumulative benefit to the watershed is so great.
There are many other types of conservation areas that may be identified through the watershed-management process. These will vary in each watershed but may include:
· groundwater recharge areas
· essential fish habitat
· drinking water reservoirs
· shellfish beds
· caves
· agricultural land
· steep slopes
· erodible soils
· 100-year floodplains
· wellhead protection areas
· shoreline areas
· mineral resources
· scenic vistas
· historic and cultural sites
· waterfowl areas
Biological Indicators of Watershed Health
Biological monitoring in streams is often chosen as a more cost-effective method of evaluating stream condition than water quality monitoring. The idea is that certain aquatic species are very sensitive to pollution or habitat disturbance, and others are very tolerant. Identifying the abundance and diversity of species present in a stream provides an indication of stream condition. In general, greater species diversity and the presence and abundance of sensitive species are indicators of good condition. If only pollution-tolerant species are present, water quality issues or other impacts are likely present (e.g., fish barriers, high flows erode in-stream habitat). Fish, benthic (bottom-dwelling) macroinvertebrates, and periphyton (algae) are commonly monitored in streams, and amphibians, reptiles, and birds may be surveyed in wetland and upland areas.
Benthic macroinvertebrates are the insects, mollusks, and worms that dwell on the bottom of a stream. They are good indicators of local water quality conditions because their migration is limited to a small area, they have a short life cycle, and they are sensitive to stress during certain stages of life. The next time you are out in the stream (preferably in spring) pick up a rock from the bottom, turn it over, and see if you can identify the bugs that are attached to it. As with all bioindicators, certain species are sensitive to pollution (e.g., mayflies, caddisflies, stoneflies), and others are either somewhat tolerant (e.g., dragonflies) or tolerant (e.g., midges, snails, aquatic worms) (DeBarry, 2004). The specific species to look for in your watershed will vary, but species lists are usually available from state and local natural heritage or wildlife departments.
There is increasing evidence that the number and species of birds present in a watershed can serve as an indicator of ecological conditions (DeBarry, 2004). Bird-habitat assessment is of great interest across the country due to the popularity of bird-watching and growing concern about the quickly declining numbers of bird species (CWP, 2002b). This information can be used as an indicator of watershed health and also to identify and conserve habitat for specific declining bird species in a watershed. Tools for conserving important habitats as part of a watershed-management strategy are discussed in module 4.
References
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