A WATERSHED is the area of land from which water drains into a given lake or river. A lake reflects its watershed because the watershed contributes both the water required to maintain a lake and the majority of the pollutants that enter the lake. Effective lake management programs must include watershed management practices. Lake problems cannot be solved without controlling the sources in the watershed.
Lake pollutants may originate from either point sources or nonpoint sources in the watershed. Point sources discharge pollutants from a distinct source such as a wastewater treatment plant or industrial facility. Point sources are usually regulated by state and federal permits.
Nonpoint pollutants include silt, nutrients, organic matter, and other substances originating over a relatively broad area. Water running over the land picks up these materials and transports them to the lake, either directly in runoff or through a tributary stream, drainage system, or ground water. Water running off a lawn or driveway during a heavy rain is nonpoint source runoff. Land uses such as agriculture, construction, and roadways contribute higher nonpoint pollutant loads than other land uses such as forests. Nonpoint pollution sources are usually controlled by implementing best management practices.
Point sources were traditionally considered to be the primary dischargers of pollution to water bodies. However, nonpoint sources (harder to identify, isolate, and control) are now more likely to be the principal contributors of nutrient and sediment loads to lakes.
Not all areas of the watershed are equal pollutant contributors. By identifying those critical areas that contribute excessive amounts of soil and nutrients to the lake, the most effective controls can be developed.
For example, agricultural runoff carrying animal wastes, soil, and nutrients can be a critical pollutant contributor. Urban runoff from lawns, gardens, streets, and rooftops may be significant sources of sediment, oils and greases, nutrients, and heavy metals to lakes. Construction and forestry activities can provide significant quantities of sediments, especially during rainstorms.
In large watersheds, the contributions from urban, forestry, and agricultural areas are generally more significant than those from lakeshore homes. In small watersheds, lakeside resident activities may be more critical pollutant contributors.
Managers of lakes and streams focus on best management practices to control four primary processes: erosion and sedimentation, stormwater runoff, nutrient inputs, and pesticides or toxic substances. These processes are highly interactive. For example, runoff control helps to reduce sediments, nutrients, and pesticide contamination in streams and lakes.
Best management practices for urban areas would include flood storage and control, street cleaning, and use of porous pavements. Such practices for construction would include stabilizing soils and limiting disturbed areas. These practices are also important to agriculture and forestry. The best place for any lake resident to use best management practices is in their own backyard.
Copyright © Washington State Department of Ecology. See http://www.ecy.wa.gov/copyright.html.