In this mechanism, sand in the river bed acts as a sink for the pollutants from the hills, when river reaches flat lands, it spreads, velocity reduces and suspended pollutants settle on the sand bed.
When river runs through hills, turbulence mixes air into water increasing the dissolved oxygen (DO). The increased DO concentration facilitates many chemical and microbiological processes in water to reduce the pollutants concentration.
Pollutants are absorbed onto sand particles, plant surfaces, rocks, etc. thereby their concentrations get reduced in the river water.
Aquatic plants and vegetation on the riverbanks absorb (nitrate, phosphate and other nutrients) and remove pesticides and heavy metals from water. In this way quality of water in the river is largely improved.
After rapid mixing of water in falls, air bubbles act as vehicles to lift many pollutants to the water surface in the form of froth (or a layer of foam). This froth is exposed to atmosphere; it facilitates oxidation of pollutants to less harmful forms. The top layer is also directly exposed to sun-light so either by increased temperature or due to various photochemical reactions; volatile organic compounds are removed from the top-layer. At different sections of river, various artificial traps help in the removal of this froth and thus rivers get self-purified.
(vii) Microbial degradation
The shallow and turbulent water results in high aeration of water. It helps in growth of bacteria and other micro-organisms. They help in river purification by microbial degradation of pollutants.