Many sewer lines are constructed next to streams to take advantage of the continuous, gradual slopes of stream valleys. Blockages, inadequate carrying capacity, leaking pipes, and power outages at pumping stations often lead to sewage overflows into nearby streams. There are three types of sewer systems:

1. Storm sewers carry storm runoff from streets, parking lots, and roofs through pipes and ditches, and eventually into streams.
2. Sanitary sewers carry raw sewage from homes and businesses to wastewater-treatment facilities.
3. Combined sewers carry a combination of raw sewage and stormwater runoff.

This picture of a sanitary sewage overflow illustrates a common problem concerning sewage overflows that occur in urban areas. Sanitary sewer overflows occur when sewer pipes clog or pumping stations break down. As shown here, raw sewage overflows from manholes and leaking pipes into nearby streams rather than backing up into homes and businesses.

Combined sewer overflows occur during storms when there is more stormwater flowing than the pipes leading to a treatment plant can handle. The excess runoff flushes human and industrial wastes, oil, toxic metals, pesticides, and litter into streams.