what is meant by self cleansing capacity of river?

The self cleaning capacity of a river refers to the process by which pollutants discharged into a river gets cleaned by natural processes. Factors like the flow of the river and the presence of bacteria which decomposes pollutants into harmless substances contribute to the self cleaning capacity of a river. Obviously, there are limits to this capacity. While a reasonable level of pollution can get cleaned through natural processes, uncontrolled pollution can be beyond the self cleaning capacity of a river.