Nonpoint source pollution is sediment (stone, gravel, and soil), excess soil fertility nutrients, and toxic substances that enter a water body from widely separated origins of its watershed, rather than from distinct origins such as a pipe or ditch.
     Many of civilization's contaminants are transported in stormwater runoff, which can enter and pollute streams, lakes, wetlands, and ground water.
     Studies of stormwater runoff and soil erosion have shown that mature forests lose less than 20 pounds of soil per acre per year and retain more than 99% of the precipitation that falls upon them. Crop land can lose 73 tons of soil per acre per year and retain 60% of the precipitation. Paved and other water-resistant surfaces retain less than 5% of the precipitation, and drain rapidly into the nearest waterbody.
     Increase in the volume and rate of stormwater discharges results in more frequent flooding and accelerated erosion of stream channels. Sediment, excess soil nutrients, and toxic substances can destroy fish habitat, decrease recreational and property values, and reduce water's potential as a drinking water source. The end result includes additional tax-dollar remediation of these effects of nonpoint source pollution.
     Sediment by weight is the largest single pollutant in the United States. Sediment reduces the productivity of aquatic plant and animal communities. It can threaten the survival of fish by covering essential spawning grounds, covering eggs, and preventing emergence of recently hatched fish. Sedimentation is a major cause of the decreased quality of fisheries throughout the United States. USDA,Chesapeake Bay Riparian Handbook.