The most common causes of pollution in rivers and streams are mud (sediment/silt), habitat changes to accommodate development, viruses and bacteria (pathogens), and nitrogen and phosphorus (nutrients). The main sources of these pollutants are stream changes to accommodate development, agricultural runoff, sewage and stormwater discharge, and construction runoff. The leading causes of pollution in reservoirs and lakes are heavy metals (like zinc and copper), dissolved oxygen (too little or too much), and chemicals like, PCBs, dioxins, and chlordane.