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The effects of urbanization on water quality:
Urban runoff

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.

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Runoff  Surface runoff and the water cycle
Effects of urbanization  Groundwater quality  Groundwater