asphalt and concrete. Although only 3 percent of the nation's land surface is designated as urban, within an urban area, the hydrological and biological changes are extreme. In Chicago, a city of 228 square miles, 45 percent of the land is now covered by impervious surfaces. The once verdant wet prairies and marshes that dominated the landscape before this great city was built are gone. The roofs, streets, and roads have greatly changed the quantity and quality of water flowing into Lake Michigan and into the Des Plaines and Illinois Rivers. The change in flow was accompanied by a dramatic change in water quality due to the large waste loads conveyed by storm water runoff and by domestic and industrial wastewater. Both the hydrologic and the water quality effects extend miles beyond the limits of the city.

The U.S. agricultural industry and urban systems have had to rely, to a great extent, on the diverse functions of aquatic ecosystems. Uplands, wetlands, and floodplains have been drained to build houses, factories, and farms. Approximately 117 million acres of wetlands alone have been lost in the United States since the 1780s (Dahl, 1990). This represents 5 percent of the total land surface in the 50 states but about 30 percent of the presettlement wetlands (excluding Alaska, the wetland loss is approximately 53 percent; Dahl, 1990). The effects of increased losses have been harmful, if for no other reason than increased flooding. The dispersive capabilities of streams and rivers were and are inadequate to handle the large amounts of runoff generated and diverted to them from uplands and former wetlands, which one acted as flood control reservoirs. In 1912, the state engineer for Illinois observed that floods on the Des Plaines River were increasing in severity and frequency (Horton, 1914). He ascribed this hydrologic phenomenon to the clearing of land and draining of wetlands in the watershed.

The widespread loss of U.S. wetlands is illustrated in Figure 1.1. When one considers the losses from 1780 to 1980 in the central United States, it is no wonder that floods ravaged the river valleys of the Ohio, Wabash, Illinois, Missouri, and Mississippi. Unfortunately, wetlands continue to be drained by ditching, and storage areas continue to be blocked by levees, so that flood damage continues to increase.

Whereas more than 60 percent of the U.S. land surface is manipulated for human needs (urban development, forests, and agricultural areas), more than 85 percent of the inland water surface area in the United States is artificially controlled (Bureau of Census, 1990). Surface water controls range from very simple fixed weirs to very complex multigated dams and extend from small farm ponds and streams to our largest rivers and the Great Lakes. They benefit us in numerous



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