CHANNEL ALTERATION

Channelization refers to the once common practice of dredging, straightening and/or widening stream channels in an attempt to reduce flooding or to lower the ground water table. The use of channelization to control flooding has been historically referred to as "stream improvement." It was given this name because the engineers who designed these projects were attempting to improve the hydraulic capacity of the stream to transport flood waters through an area. This was done using a number of different approaches, including: widening the stream channel so it would hold more water, building berms along the edges to the stream to hold the flood flow in the channel, straightening the stream to increase the slope of the water to move it faster through an area and/or reducing the roughness of the stream channel by constructing a smooth channel out of concrete. In addition to flood control projects, channelization has also been done in some areas to help lower the ground water table to drain adjacent wetlands and cropland.

While channelization can be partially effective at reducing flooding or lowering the ground water table in an area, it can also have a variety of negative environmental impacts. Channelized steams often have poor in-stream habitat for aquatic organisms, they can be a barrier to fish migrations and in areas where the riparian buffer has been removed, the water in the stream can be heated by the sun during the day reducing its oxygen holding capacity and raising water temperatures above the tolerance limits of some fish species. In addition, while channelization may be able to reduce flooding in one specific stream reach, often it increases flooding downstream.

In the past, channelization was a common practice in many areas. For example, in one Maryland county there is estimated to be more than 100 miles of stream that have been replaced with concrete trapezoid channels. Fortunately, because of the high cost, limited benefits, and significant environmental impacts, widespread stream channelization is not done any more. In fact, in recent years there have even been several projects in Maryland to remove concrete channels and restore them to a more natural stream shape.

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This page was updated on 4/8/2004