Many everyday activities create waste. Even cutting the lawn makes grass clippings that can stress water bodies such as streams and rivers if too much washes into the water. This is because the grass is food for in the water. Microorganisms can grow quickly in great numbers, and rob the water of the oxygen that is needed by fish and other living creatures inhabiting the water.

One person alone might not produce enough waste material to contaminate a water source. However, a community of many people and large industries produce large amounts of waste. For example, the city of Halifax, Nova Scotia, on Canada's east coast still directly discharges sewage into the harbor. Even though the city is relatively small (approximately 300,000 people), millions of gallons of raw sewage pour into the water each day. If enough waste gets into a stream, river...