Inland surface water: Chapter 18

Freshwater aquatic ecosystems include rivers and streams, large and small lakes, reservoirs, and ephemeral ponds. Wetlands are defi ned and discussed in Chapter 17 of this report. It is estimated that there are 123,400 lakes with a surface area greater than 4 ha in the United States. Most lakes, however, are smaller than 4 ha; small lakes account for the majority of lake surface area both globally and in the United States (Table 18.1; Downing et al. 2006). Th e density of lakes varies greatly by region of the country, from 8.4 lakes per 100 km2 in the upper Midwest and 7.8 lakes per 100 km2 in Florida, to much lower values in other areas of the country (e.g., mid-Atlantic, Southeast, and West <1.0 lakes per 100 km2 ) ( Eilers and Selle 1991). Th e cumulative surface area of these lakes is approximately 9.5 million ha. Th e U.S. Geologic Survey's National Hydrographic Dataset (NHD) estimates that there are approximately 1.1 million km of perennial fl owing streams in the United States. Of these about 91 percent are fi rst through fourth order (“wadeable”) (US EPA 2006).