They absorb heavy metals from the water and store them in plant tissues. N and P are the ingredients in fertilizer, and do great damage to the Chesapeake Bay. N and P pollution comes from agricultural areas, golfcourses, lawns and other landscaped areas when more fertilizer is used than is necessary for the plants. Tree roots prevent soil erosion. The mesh of deep roots holds onto the soil so that the stream waters do not wash it away.
Soil erosion makes the water cloudy with sediment.
When the water is cloudy, plants in the water do not get enough sunlight. Fish gills get clogged with sediment when they breathe cloudy water. Cloudy water is one of the main causes of environmental decline in the Anacostia River and the Chesapeake Bay. Streamside forests shade the stream. Without shade, stream water temperatures can rise 5 to 10 degrees Farenheight.