The area of land draining into a stream is called the stream's watershed. Biological activities in the watershed such as nutrient and water consumption by vegetation and the retention of soil by roots regulate the supply of water, nutrients and sediment particles to streams. Disruption of normal biological activities due to the removal of forests can increase the supply of water, sediment particles and nutrients to streams and thus degrade the stream's water quality. Agricultural activities can increase the degradation of stream water quality by increasing the likelihood of erosion and by adding fertilizer, animal wastes, and herbicides to the landscape. Increased nutrient concentrations can stimulate the growth of algae and plants in streams to unhealthy levels that have negative consequences for stream invertebrates and fish.
Herbicides can be toxic to aquatic plants and animals.
Because streams are important sources of inputs for many lakes, increased suspended sediment, nutrient levels and herbicides in streams can also cause problems for the water quality of lakes. The impact of agriculture on the water quality of streams should vary seasonally. 2. during storm events when there is a pulsed increase in water runoff. While the input of sediment and nutrients is highest in early spring and late fall, the enhancement of inputs by agriculture is probably largest in late spring and summer. In late spring, plowing and herbicide treatment of fields can significantly increase inputs of sediment particles and chemical pollutants into agricultural streams.