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26 RESTORATION OF AQUATIC ECOSYSTEMS TABLE 1.2 Water Use in the 50 States for 1985 (million gallons per day) Off-stream On-stream Sector Ground Surface Total Con- Re- sumed turned Domestic-commercial 3,989 31,311 35,300 6,884 28,417 Industrial-mining 5,267 25,533 30,800 4,928 25,872 Thermoelectric 655 130,345 131,000 4,323 126,677 Irrigation-livestock 48,504 92,496 141,000 75,999 65,001 Hydropower 3,050,000 Total 58,415 279,685 338,100 92,134 245,967 3,050,000 SouxcE: Solley et al., 1988 it moves through river systems. Given that there-are well over 2.5 million dams in the United States (Johnston Associates, 1989), only a small probability exists that a drop of water could make its way from its cloud of origin, over the land surface, through the drainage sys- tem, and back into an ocean without passing through aman-made structure. Both off-stream and on-stream uses change the physical and chemical characteristics of the water. Reservoirs alter the thermal properties of the waters in rivers and streams by changing the surface area and :depth characteristics. During the winter the larger surface areas cre- ated by a reservoir release more heat than an undammed stream would have, whereas during the summer they absorb more heat; con- sequently, the downstream thermal regime is changed. Thermal electric plants discharge heat to streams, rivers, and lakes via the dispersal of cooling waters: Domestic and industrial (including thermal electric)- uses alter the hydrology at the point of both withdrawal and dis- charge. The return flows introduce elevated concentrations of nutri- ents and toxic substances despite modern wastewater treatment tech- nology. Relative to the sustained yield, industrial and domestic wastewaters represent about 32 percent of the water treated. Dis- solved solids are added to the stream from irrigation return flows and agricultural drainage in general. These flows account for 12 percent of the sustained yield. The high concentrations of dissolved solids result, in part, from the evaporation of irrigation water. Evap- orative losses account for 14 percent of the sustained yield. Other sources such as runoff from roads, parking lots, and farm fields contribute substantial amounts of solids and nutrients to our rivers, lakes, and streams.