Some human activities are harmful to rivers:
Pollution does not just affect the creatures that live in the water. When people eat fish taken from polluted streams, the pollution passes into their bodies and can cause cancer or other health problems.
Water is usually cleaned before it is piped into people's homes. This involves: screening to remove any large objects, clarification where a chemical is added to cause particles to clump together, filtration through sand and gravel beds and disinfection often using chlorine.