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Why Water Quality Changes

Rivers, lakes, and oceans are not sterile bodies of water. They contain naturally occurring organisms and bacteria and can be contaminated by outside sources. The most frequent outside sources of microbial contamination are:

  • Polluted stormwater runoff
  • Sewage overflows
  • Boating wastes

Contamination in large bodies of water is often highest during and immediately after rainstorms. Rainwater picks up wastes and other pollutants as it runs off lawns, farms, streets, and other ground sites and into streams.

Learn how to prevent water pollution

Rainfall and River Water Quality

Most of the contamination in the Schuylkill River originates upstream from the City of Philadelphia in the Schuylkill River Watershed. Water quality in a river changes because it is affected by many factors including weather, climate, rainfall, industrial and sewage discharges, and accidental spills. It is strongly impacted by runoff from rain events. This is easily observed by the eye during and after storms when streams and rivers are very cloudy and look brown.

When it rains, the dirt, animal waste, and other contaminants that build up on the ground surface (e.g., pavement) are washed off into streams and rivers. Though there is more water in streams and rivers during storms, there are more contaminants as well.

Example: Stream Water Quality Changes During a Storm

In this hypothetical example, bacteria levels in the stream (the green dots) increase during a storm event and correspond with the rise in the water level (the blue line) and increase in stream flow during the rain event.