<P> As it flows, the amount of runoff may be reduced in a number of possible ways: a small portion of it may evapotranspire; water may become temporarily stored in microtopographic depressions; and a portion of it may infiltrate as it flows overland . Any remaining surface water eventually flows into a receiving water body such as a river, lake, estuary or ocean . </P> <P> Urbanization increases surface runoff by creating more impervious surfaces such as pavement and buildings that do not allow percolation of the water down through the soil to the aquifer . It is instead forced directly into streams or storm water runoff drains, where erosion and siltation can be major problems, even when flooding is not . Increased runoff reduces groundwater recharge, thus lowering the water table and making droughts worse, especially for farmers and others who depend on the water wells . </P> <P> When anthropogenic contaminants are dissolved or suspended in runoff, the human impact is expanded to create water pollution . This pollutant load can reach various receiving waters such as streams, rivers, lakes, estuaries and oceans with resultant water chemistry changes to these water systems and their related ecosystems . </P> <P> A 2008 report by the United States National Research Council identified urban stormwater as a leading source of water quality problems in the U.S. </P>

Rivers obtain water from the surface run off and ground water of the drainage basin