Your Paper should be three to four text-written pages, plus one page of endnotes and one page that shows a map of the river basin

Chapter 14 River Systems 433

because offshore currents remove sediment as quickly as it is deposited.

Floods Despite our historical knowledge of flood events and their effects, floodplains continue to be important sites of human activity and settlement. These activities place lives and property at risk during floods, especially in less-developed regions of the world. Bangladesh is per- haps the most persistent example: It is one of the most densely populated countries on Earth, and more than three-fourths of its land area is a floodplain and delta complex—an area the size of Alabama. The historic floods of 1988 and 1998 inundated 60% and 75%, respectively, of the country’s land area, causing extensive crop losses and thousands of fatalities.

A f lood is defined as a high water flow that passes over the natural bank along any portion of a stream. As discussed earlier, floods in a drainage basin are strongly connected to precipitation and snowmelt, which are, in turn, connected to weather patterns (Fig- ure 14.32). Floods can result from periods of prolonged rainfall over a broad region, from intense rainfall as- sociated with short-lived thunderstorms, from rapid melting of the snowpack, or from rain-on-snow events that accelerate snowpack melting. Floods vary in mag- nitude and frequency, and their effects depend on many factors.

Flood Probability Maintaining extensive historical records of discharge during precipitation events is critical for predicting the behavior of present streams under similar conditions. The U.S. Geological Survey has detailed records of stream discharge at stream-gaging stations since the 1900s, with

Recurrence Interval, in Years

Probability of occurrence in a Given Year

Percent Chance of occurrence in a Given Year

10 1 in 10 10

50 1 in 50 2

100 1 in 100 1

500 1 in 500 0.20

1000 1 in 1000 0.10

table 14.2 Recurrence Interval and Probability of Occurrence for Flood Discharges

the most consistent data collected since the 1940s. These relatively short-term historical data form the basis for flood probability estimates.

Recurrence Interval Scientists rate flood discharges statistically according to the recurrence interval (or return interval), the estimated time interval between peak discharges of similar size. For example, based on dis- charge data for a particular stream, a “100-year flood” on that stream has a recurrence interval of 100 years and a 1% chance of occurring in any given year (Table 14.2). The use of historical data works well where available; however, urbanization and dam construction can change the magnitude and frequency of flood events on a stream or in a watershed.

These statistical estimates are probabilities that events will occur randomly during a specified period; they do not mean that events will occur regularly during that time period. For example, several centuries might pass without a 100-year flood, or a 100-year level of flooding could occur twice in one century.

Annual Exceedance Probability Another method for describing floods and precipitation events uses the annual exceedance probability (AEP) to represent the statistical likelihood of occurrence. By this measure, a 100-year flood has a 1% AEP.

WoRkitOut 14.4 Recurrence of Rainfall and Flooding

News reports about the 2016 West Virginia flooding pictured in Everyday Geosystems (Figure 14.1) described it as being caused by a “thousand-year precipitation event.”

1. What is the percent chance of a rainfall event of this magni- tude occurring in any given year?

2. Could a precipitation event of that magnitude occur again in your lifetime?

3. Does a 1000-year rain event produce a 1000-year flood event? Explain.

▲ Figure 14.32 Flooding from Hurricane Matthew in 2016. Rising floodwater caused by heavy rainfall from Hurricane Matthew inundated portions of North Carolina in October 2016; shown here is the town of Rocky Mount, flooded by the Tar River. [Thomas Babb/The News & Observer via AP.]

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434 Geosystems

Floodplain Risk The flood recurrence interval is useful for floodplain management and hazard assessment. A 10-year flood in- dicates a moderate threat to a floodplain. A 50-year or 100-year flood is of greater consequence, but it is also less likely to occur in a given year.

Scientists and developers define and map flood- plains using flood recurrence intervals, for example, by delineating the “100-year floodplain.” Using these maps, scientists and engineers can develop the most effective flood-management strategy. Restrictive zoning using these floodplain designations helps determine degrees of risk across the floodplain and can help avoid potential flood damage. However, restrictive zoning based on flood hazard mapping is not always enforced.

Flood Protection In the United States, floods cause an average of about $6 billion in annual losses. The catastrophic floods along

the Mississippi River and its tributaries in 1993 and 2011 produced damage that exceeded $30 billion in each oc- currence. Flood protection, when in place, generally takes the form of dams (discussed in Chapter 8) and arti- ficial levee construction along river channels.

Usually, the term levee connotes an element of human construction, and these engineered features are common across the United States and throughout the world. Artificial levees are earthen embankments, often built on top of natural levees. They run parallel to the channel (rather than across it, like a dam) and increase the capacity in the channel by adding to the height of the banks (Figure 14.33). For efficient use of time and mate- rials, channels are often straightened during levee con- struction. Levees are intended to hold floods within the channel, but not prevent them completely. Eventually, given severe enough conditions, an artificial levee will be overtopped or damaged in a flood. When overtopping (known as levee breaching) or levee failure occurs, exten- sive flood damage and erosion can result downstream.

(b) Sheep graze on the slopes of an artificial levee along the Sacramento River in California. Note that the agricultural fields are lower in elevation than the river, caused by subsidence of the Sacramento River delta.

(a) A natural levee. Natural levee

(c) The MIssissippi River flows over part of an intentional breach in the Bird's Point levee in Missouri in 2011. During the winter floods of early 2016, 11 levees were breached nearby as the Mississippi crested to near-record levels in the U.S. Midwest.

georeport 14.2 America’s levees By several estimates, over 100,000 miles of artificial levees exist along rivers and streams in the United States, the vast majority of them

privately owned. The U.S. population living in areas protected by levees is estimated to be in the tens of millions; some major urban areas with levee systems are New Orleans, Sacramento, Dallas–Fort Worth, St. Louis, and Washington, D.C. In fact, over 30 major cities in America are lo- cated on floodplains. Currently, no national policy exists concerning the safety of levees (see http://www.leveesafety.org/docs/NCLS-Recom- mendation-Report_012009_DRAFT.pdf).

▲Figure 14.33 Natural and artificial levees. [(b) California Department of Water Resources. (c) Scott Olsen/Getty Images News.]

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In 2011, Americans spent $42 million on fishing-related activities. Streams in Montana, Missouri, Michigan, Utah, and Wisconsin are of high enough quality that they are designated “blue ribbon fisheries” based on sustainability criteria such as water quality and quantity, accessibility, and the specific species present. [Karl Weatherly/Getty Images.]

After days of heavy rain, the Seine River reached its highest flood stage in over 30 years in Paris, France, in June 2016. High water closed rail lines, the Metro system, numerous tourist attractions, and all boat traffic through the city. [Joel Saget/AFP/Getty Images.]

A proposed series of dams on the free-flowing Nu/Salween River system in Southeast Asia would relocate some 60,000 people in China. The dams would also block the movement of sediment that replenishes farmlands along the river’s floodplain and delta. [Bradley Mayhew/Getty Images.]

A Texas Department of Safety boat patrols the U.S.–Mexico border along the Rio Grande in Texas for drug trafficking and human smuggling activities. The border follows the center of the river and was surveyed and permanently established to avoid disputes related to channel changes. [Polaris/Newscom.]

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14b

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RIVER SYSTEMS IMPACT HUMANS • Humans use rivers for recreation and have farmed fertile floodplain soils for centuries. • Flooding affects human settlements on floodplains and deltas.

HUMANS IMPACT RIVER SYSTEMS • Dams and diversions alter river flows and sediment loads, affecting river ecosystems and habitat. River restoration efforts include dam removal to restore ecosystems and threatened species. • Urbanization, deforestation, and other human activities in water- sheds alter runoff, peak flows, and sediment loads in streams. • Levee construction affects floodplain ecosystems; levee failures cause destructive flooding.

ISSUES FOR THE 21ST CENTURY • Increasing population will intensify human settlement on floodplains and deltas worldwide, especially in developing countries, making more people vulnerable to flood impacts. • Stream restoration will continue, including dam decommissioning and removal, flow restoration, vegetation reestablishment, and restoration of stream geomorphology. • Global climate change may intensify storm systems, including hurricanes, increasing runoff and flooding in affected regions. Rising sea level will make delta areas more vulnerable to flooding.

QUESTIONS TO CONSIDER 1. How do human activities affect river systems? Try to think of both negative and positive impacts. 2. What hazards will human populations on floodplains and deltas face during the 21st century?

TheHumandenominator 14 Rivers, Floodplains, and Deltas

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