Channelization
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This article is about stream channelization. For channelization in the context of telecommunications, see Channelized.
Channelization is the process of reconstructing the natural course of a stream in order to make it flow into a restricted path.
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[edit] Channelization around the world
Extensive channelization of streams has occurred in much of the Midwestern United States and the Southern United States, primarily for purposes of agricultural development. Much of this was done under the auspices or overall direction of the United States Army Corps of Engineers. One of the most heavily channelized areas in the United States is West Tennessee, where every major stream with one exception (the Hatchie River) has been partially or completely channelized.
[edit] Advantages
Channelization of a stream may be undertaken for several motives. One is to make a stream more suitable for navigation or for navigation by larger vessels with deep draughts. Another is to restrict water to a certain area of a stream's natural bottom lands so that the bulk of those lands can be made available for agricultural purposes. A third reason given is flood control, with the idea of giving a stream a sufficiently large and deep channel that flooding beyond those limits will be minimal or nonexistent, at least on a routine basis. One major reason is to reduce natural erosion; as a natural water way curves back and forth, it usually deposits sand and gravel on the inside of the corners where the water is flowing slowly, and cuts sand, gravel, subsoil, and precious topsoil from the outside corners where it is flowing fast due to a change in direction. Unlike sand and gravel, the topsoil that is eroded does not get deposited on the inside of the next corner of the river. It simply washes away. The channelization of a water way by straightening it prevents the water from changing directions randomly, the net erosion is greatly reduced.
[edit] Disadvantages
Channelization has several predictable and negative effects. One of them is loss of wetlands. Wetlands are an excellent habitat for many forms of wildlife, and additionally serve as a "filter" for much of the world's surface fresh water. Another is the fact that channelized streams are almost invariably straightened. For example, the channelization of Florida's Kissimmee River has been cited as a contributing cause of wetlands loss. This straightening causes the streams to flow more rapidly, which can, in some instances, vastly increase soil erosion. It can also increase flooding downstream from the channelized area, as larger volumes of water travelling more rapidly than normal can reach choke points over a lesser period of time than they otherwise would, with a net effect of flood control in one area coming at the expense of greatly aggravated flooding in another.
[edit] Modern policy
For the reasons cited above, in recent years in the U.S. stream channelization has been greatly curtailed, and in some instances even partially reversed. The United States Government now has in place a "no net loss of wetlands" policy that means that stream channelization in one place has to be offset by the creation of new wetlands in another, a process known as "mitigation". The major agency involved in the enforcement of this policy is the same Army Corps of Engineers which for so long was the primary promoter of wide-scale channelization. Often, in the instances where channelization is permitted, boulders may be installed in the bed of the new channel so that water velocity is slowed, and channels may be deliberately curved as well.
This new policy also has its critics, however. Farmers who are losing land to soil erosion as channelized streams cease to be maintained feel particularly aggrieved. Not only are they losing their valuable property to erosion, the erosion ends up in the stream or river, and contributes to decreased water quality. They also point out that if such policies had been in place in the U.S. at the time of white settlement that the country probably would never have become a leader in world agriculture and a net exporter of food. Channelization critics respond that this is immaterial, as we no longer are living in the era of initial settlement. This issue will be debated into the foreseeable future.
