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Introduction The process of channel modification has played a major, although not always beneficial, role in the development of this country. Land drainage has been necessary to convert swampland into fertile, productive farmland. Dredging of our stream bottoms has led to the discovery of precious metals and also to the creation of navigable waterways to transport our people and products. As our cities developed, it was often found that the river that provided a ready source for water supply and waste disposal was also likely to periodically carry damaging flood waters. Hence, the need arose to redesign and, in some cases, relocate these streams. Channel realignment has also been necessary in numerous instances to provide suitable bridge crossings and right-of-ways for our highway system. Overgrazing of the riparian communities bordering our rivers and streams has led to possibly more subtle but nonetheless damaging impacts. In total, Arthur D. Little (1973) estimated that by 1972, over 200,000 miles of stream channel had been modified in the United States.
Given the sheer magnitude of such river manipulations and an increasing awareness by the public of the environmental ramifications of such acts, it is little wonder that engineers and biologists find themselves continually debating the pros and cons of channel modification. Whether it is called channelization, improvement, alteration, realignment, or stabilization, there will be definite impacts to the specific stream reach involved as well as possible upstream and downstream effects. Such impacts can be positive or negative depending upon the nature of the modification and the nature of one's interest in the river reach undergoing change. Potentially, the following characteristics of a reach could be altered:
Historically, of the characteristics listed above, consideration was typically given only to the first two, and possibly the third. This has changed dramatically however, in recent years, as the concept of river restoration has become more widespread. Nunnally (1978) stated, regarding river restoration:
Many of the detrimental effects of channelization can be avoided, with little compromise in channel efficiency, by employing channel design guidelines that do not destroy the hydraulic and morphologic equilibria that natural streams possess. These guidelines include minimal straightening; promoting bank stability by leaving trees, minimizing channel reshaping, and employing bank stabilization techniques; and, emulating the morphology of natural stream channels.
The underlying tenent of the river restoration approach is that by thorough planning done before modification activity begins, a design simulating that of nature as closely as possible can be developed that not only alleviates the problem causing the needed modification, but also preserves many of the other valued reach characteristics. Too often in the past, the preservation of fish habitat, for example, was given little or no consideration until after the modification was completed. Later, when population levels were found to be declining due to the loss of habitat, attempts were made to artificially increase the carrying capacity of the reach by the addition of a variety of improvement structures. This is not to say that there is no place for structures such as wing deflectors or bank covers in habitat management. Rather, the point is that if proper planning had occurred during the design process, the need for these structures may not have been so great. From a fisheries standpoint, a most simplistic view of the channelization process and associated impacts could be illustrated by the following flow diagram:
D Land and/or --> D Channel --> D --> D Habitat --> D Population
Stream Use Morphology
where D = change in
--> = leads to
The key to the river restoration approach is for the habitat biologist to have input
into the process prior to a change in channel morphology brought about by modification,
rather than after the habitat and population changes have already occurred.
Generally, the organization of this chapter will follow the progression shown in
the above diagram. After a brief review of the basic in-stream components of
fish habitat (for brevity, this will focus on the salmonid family), the impacts of
various channel modification activities on habitat diversity will be discussed. The
concluding section of the chapter will then concentrate on channel restoration
procedures and structures to enhance fish habitat, from a planning aspect as well
as from a design and installation approach.
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