Runoff is water - from rain, snowmelt, or other sources - that flows
over the land surface. Runoff is a component of the water cycle. If
runoff occurs on surfaces before reaching a drain it is also known as
overland flow. An area which produces runoff draining to a common point
is called a watershed. When runoff flows along the roof, pavement and
ground, it will pick up contaminants such as deposited air pollution,
plastic litter, cigarette butts, animal wastes, petroleum, pesticides
and fertilizers that are then discharged to sewers, and/or, local
streams, lakes and oceans. Urban areas have more surface runoff due to
impervious surfaces such as pavement and roofs that do not allow
percolation of the water down through the soil to the aquifer. Because
of the impermeable surfaces runoff is funneled into storm sewers,
ditches and streams, where pollutants, erosion and siltation can case
beach closures, fish kills and threats to wildlife. Increased runoff
reduces groundwater recharge, thus lowering the water table, as well as
reducing water flow to local streams and rivers and making droughts
worse, especially for farmers and others who depend on water wells.