What is the largest source of water pollution?
The most important contamination sources that cause impairment of water quality in rivers and streams are (1) Agriculture (48%), (2) hydrological changes/modifications (20%), (3) habitat modification (14%) and urban run-off/storm severs (13%) (US EPA, 2002).
Similarly, for lakes, agriculture was found as #1 sector in terms of water pollution (41%). The other sources are hydrologic modification (18%), urban run-off/storm severs (18%), and nonpoint sources (14%) (US EPA, 2002).
US EPA (2002). National Water Quality Inventory: 2000 Report. Environmental Protection Agency, Office of Water, EPA-841-R-02-001. Washington, D.C., USA.
Just a quick edit:
Agriculture is the #1 source because mainly of the use of fertilizers. When these fertilizers run into water sources, the water becomes nutrient rich, and algae commonly bloom in great numbers. This depletes the oxygen used by fish and other aquatic life in the stream. Another form of this is cattle feces getting into the water, which has the same effect.