USGS - science for a changing world

The USGS Water Science School

How urbanization affects
the hydrologic system

People began to build towns and cities as soon as they came to America. The speed of this urbanization picked up after World War II, and now many urban areas are growing at a record pace. What are the effects on the local hydrologic system when a rural area is turned into an area full of housing developments, shopping centers, industrial buildings, and roads?

In the table below, the left side shows changes in land and water use when urbanization occurs, and the right side shows the possible effect on the local water system.


Beginning of urbanization

Change in Land Use:
Remove trees and vegetation. Begin building houses, some with sewers and some with septic tanks. Begin drilling wells.
Effect on Water System:
More storm runoff and erosion because there is less vegetation to slow water as it runs down hills. More sediment is washed into streams. Flooding can occur because water-drainage patterns are changed.

Beginning of large-scale urbanization

Change in Land Use:
Bulldozing of land for houses and subdivisions. Filling in of farm ponds. Building of roads. Diverting streams to supply water for people. Discharging of sewage into streams.
Effect on Water System:

More land erosion and more sediment is washed into streams. This increases the chance of flooding and harms the water quality of the streams. Local flooding can occur. Some small streams are paved over (using culverts). Natural land that used to soak up runoff is replaced by roads and large areas of pavement (impervious areas) . This means that the water that used to soak into the ground now runs off into streams. The runoff can also be collected by storm sewers and sent to small streams, which can flood.

Increased sewage in streams causes pollution -- it can kill fish and make water unusable for other purposes downstream.


Continuing urbanization

Change in Land Use:
Urbanization is finished by the addition of more roads, houses, and commercial and industrial buildings. More wastewater is discharged into local streams. New water-supply and distribution systems are built to supply the growing population. Reservoirs may be built to supply water. Some stream channels are changed to accommodate building construction. Industries might drill some deep, large-capacity wells.

Effect on Water System:
More pavement means less water will soak into the ground, meaning that the underground water table will have less water to recharge it. This will lower the water table. Some existing wells will not be deep enough to get water and might run dry.

The runoff from the increased pavement goes into storm sewers, which then goes into streams. This runoff, which used to soak into the ground, now goes into streams, causing flooding. Changing a stream channel can cause flooding and erosion along the stream banks. More sewage is discharged into streams that weren't "designed by nature" to handle that much water.

The use of too many large wells can lower the underground water table. This can cause other wells to run dry, can cause saltwater to be drawn into drinking-water wells, and can cause land that was formerly "held up" by underground water to subside, resulting in sinkholes and land subsidence.

Local community takes steps to fix some problems

Change in Land Use:
Improvements in the storm drainage system are made. Wells are drilled to recharge underground aquifers. Projects to reuse wastewater might be started.
Ecological-designed recharge ponds disperse some storm drainage to artificially recharge shallow aquifers.
Effect on Water System:
New storm-drainage systems reduce flooding during storms. Less damage is done to basements, yards, and streets. Water is actually injected into recharge wells to put water back into underground aquifers. Reusing wastewater means less pollution, more water conservation, and additional water for recharging aquifers.

Related topics:

Urbanization and water quality  Impervious surfaces and flooding

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Page Last Modified: Monday, 17-Mar-2014 11:03:39 EDT