Much of the rainfall in watersheds having forests
and pastures is absorbed into the porous soils (infiltration), is
stored as ground water, and moves back into streams through seeps and
springs. Thus, in many rural areas, much of the rainfall does
not enter streams all at once, which helps prevent flooding.
When areas are urbanized, much of the vegetation and top soil is replaced by impervious surfaces such as roads, parking lots, and pavement (picture at left). When natural land is altered, rainfall that used to be absorbed into the ground now must be collected by storm sewers that send the water runoff into local streams. These streams were not "designed by nature" to handle large amounts of runoff, and, thus, they can flood.
Drainage ditches to carry stormwater runoff to storage ponds are often built to hold runoff and collect excess sediment in order to keep it out of streams.
So, how can excessive urban runoff harm streams?
Water running off of impervious areas, such as roads and
parking lots, can contain a lot of contaminants, such as oil
and garbage. This runoff often goes directly into streams.
Following summer storms, runoff from heated roads and
parking lots causes rapid increases in stream temperatures that can produce thermal
shock and death in many fish.
Use of stormwater impoundments and porous paving materials
can reduce stormwater runoff and the movement of contaminants from
roads and other areas to streams.
Regulations and controls on the location and amount of
impervious area can lessen the damage that contaminants
can do to streams.
Runoff of sand and salt used to help remove snow from roads
can contaminate streams.
U.S. Department of the Interior | U.S. Geological Survey
URL: http://ga.water.usgs.gov/edu//urbanrun.html
Page Contact Information: Howard Perlman
Page Last Modified: Wednesday, 13-Aug-2008 07:22:36 EDT
