‘’’Urban runoff’’’ is
surface runoffof rainwater created by urbanization. In this case, instead of percolating through soil, it runs over impermeable materials such as asphalt, cement, and concrete, and is forced into storm drains. This causes lowering of the water table(because groundwater rechargeis lessened) and flooding since the amount of water that remains on the surface is greater [http://www.epa.gov/nps/facts/point7.htm Managing Urban Runoff | Polluted Runoff (Nonpoint Source Pollution) | US EPA ] ] .
rainwaterfrom percolating into the ground, thus turning rainwater into dangerous stormwater. Stormwater is channeled into storm drainsand into rivers. This decreases the sediment load, but increases the flow. In fact, a typical city blocks nine times the groundwater percolation than a typical woodland. Water running off these impervious surfaces tend to pick up oil, heavy metal and garbage, which is sent directly into rivers via storm drains. The runoff also increases temperatures in streams, harming fish. (A sudden burst of runoff from a rainstorm can cause a fish-killing shock of hot water.) Also, salt used to melt snow picked up from roads can contaminate streams [ [http://ga.water.usgs.gov/edu/urbanrun.html Urbanization/water quality: Urban runoff ] ] .
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