This slowdown enables a number of pollution-attenuating functions to occur. Much if not most of the runoff filters into the soil within the riparian area, where sediments are trapped and where excess nutrients, heavy metals and many other pollutants either adhere to or are taken up and sequestered in living plant tissue or are broken down into less harmful substances by soil bacteria and other microorganisms. A similar phenomenon occurs in the river itself, where sediments are held back by the large woody debris, which also provides ample surface area to support a large population of microbes that consume excess nutrients and other pollutants that have already gotten into the water. Without the protection afforded by streamside forests, rivers and streams are vulnerable to being degraded by a host of pollutants, many of which pose specific problems relating to public and private water supplies. Excessive sediments carried into rivers by stormwater and other means harm water supplies by damaging water treatment pumps and other equipment, increasing treatment costs to remove the sediment, and reducing reservoir storage capacity.
Sediment can also decrease river bottom infiltration, reducing the yield of nearby wells.
In addition, chlorine is generally less effective as a water treatment disinfectant if the water has a high level of suspended sediment particles. A related problem is that removing streamside forests leads to an increase in water temperature and sunlight, both of which can contribute to increased algae growth. Metals coming from urban runoff have several adverse impacts. First, they increase the costs of treatment to remove the metals. Second, they also can form deposits in pipes, reducing their carrying capacity.