percent of the wetlands remaining in 1950 have since been converted to another use (see also Table 1-1). In addition, less than 2 percent of the nation’s 3.1 million miles of rivers and stream remain free flowing for longer than 125 miles and include more than 75,000 dams larger than 6 feet and 2.5 million smaller dams (TNC, 1998). Within the United States, more than 60 percent of freshwater mussels and crayfish are considered rare or imperiled and 35 percent or more of fish and aquatic amphibian species are at some risk of extinction (Abell et al., 2000). Thus, the number and amount of intact functional aquatic ecosystems have been substantially reduced in recent decades. This relative scarceness has called increasing attention to the need to better understand the functionality and value of the remaining ecosystems to society.

Despite the large losses and changes in these systems, aquatic ecosystems remain broadly and heterogeneously distributed across the nation. At a glance, there are almost 4 million miles of rivers and streams, 59,000 miles of ocean shoreline waters, and 5,500 miles of Great Lakes shoreline in the United States (EPA, 2002). There are 87,000 square miles of estuaries, while lakes, reservoirs, and ponds account for more than 40 million acres. As of 1997, the lower 48 states contained about 165,000 square miles (105.5 million acres) of wetlands of all types—an area about the size of California (Dahl, 2000). Figure 1-1 shows major rivers and streams. Figure 1-2 shows major aquifers in the United States classified by major features that affect the occurrence and availability of groundwater. A variety of federal programs report on the extent, status, and related trends of aquatic ecosystems located throughout the United States. Although it is beyond the scope of this report to review systematically or even summarize all such programs, a few of the largest and most important programs are described briefly in Chapter 3.