REHABILITATION CONCEPTS

A natural stream is a balanced ecosystem that has evolved over many years. Innumerable floods have molded the channel into a form that changes little from year to year, and vegetation has grown up to protect the banks and shade the water. Interdependent communities of plants and animals occupy the stream, each species finding the habitat best suited to its existence.

This balanced condition is upset by stream relocation, and under the best conditions its restoration will take a number of years. If the relocation is badly done, the stream may never fully recover. Stream relocation design should strive to establish the basic conditions that will favor rapid recovery by natural processes. First among these conditions is channel stability.

Channel Stability Is Essential for Good Fish Habitat

A prime cause of channel instability is shortening the length of the stream. This may start a cycle of erosion that can damage the stream both upstream and downstream from the relocated section. The best treatment for stream relocations is to preserve the original gradient throughout the relocation. This can be done by making the new channel the same length as the old one. If the channel must be shortened, the original gradient can be preserved by building check dams to absorb some of the elevation difference. In some cases, the channel may be made rougher to offset the steeper gradient. Habitat improvement measures such as large rocks can sometimes serve this purpose.

Bank erosion is an essential part of the natural channel-forming process, and should be expected in all newly-excavated channels. However, erosion may have to be controlled in places to prevent damage to the highway or adjoining property. This can best be accomplished by riprap bank protection.

Stream Relocations Should Have a Natural Appearance

The new channel of a relocated stream should appear as much like the adjoining undisturbed portions of the stream as possible. -The new channel should have a somewhat curved alignment, and should vary in width and depth. The slopes should vary in steepness to simulate the irregular shapes of nature. This cannot be accomplished by man's efforts alone. At best, all he can do is to excavate a large enough channel, and then let the stream do the rest of the work of naturalization, assisted by whatever habitat measures may be appropriate.

The new channel should be helped to recover its garment of vegetation. During construction, no original vegetation should be unnecessarily removed or damaged. Large trees can sometimes be preserved by changing the alignment of the new channel or by steepening the side slopes. By leaving much of the well-established existing vegetation in place, the stream esthetics and a large degree of bank stability can be preserved without a long wait for new vegetation to grow.

After the new channel is graded the banks and back-slopes should be planted with shrubs and grasses to quickly restore a natural appearance and reduce bank erosion. Nearby native plant varieties should be used if possible. It may also be desirable to plant young trees to speed up the recovery process.

Rocks and structures placed in new channels to develop fish habitat should have a naturalappearance and blend into the scenery. Riprapped slopes should be partially covered with soil to provide a toe hold for grasses and plants, which then mask the harsh artificial rock slopes to some extent. Trees can be planted in cells of soil built into the riprap at protected places. Since cattle and horses can cause heavy damage to streamside vegetation, they should be fenced out of new plantings.

Habitat Improvement

A newly-excavated channel lacks many of the features of a good fish habitat. However, in time the annual floods will form the bed materials into pools and riffles, trees and bushes will grow on the banks, and the habitat will gradually improve. This slow process can be hastened by special measures. Seeding and transplanting trees and bushes can help to restore the bank vegetation which provides shade and hiding cover for the fish. Placing rocks, gravel, or special structures in the channel will help the stream to more rapidly shape its bed in a manner favorable for fish habitat.

Occasionally, it may be necessary to relocate a stream in which no fishery exists, due to poor water quality or past relocations. In such streams, measures to restore fish habitat may not be worthwhile. Even so, if efforts are under way to clean up the stream, or if the potential for a game fishery exists, it may be desirable to install habitat structures for future use.

A braided stream may change its channel abruptly during floods. Habitat improvements are seldom effective in such streams, as the channel may shift elsewhere and leave them high and dry.

Streams that approach zero flow at times due to diversion or natural conditions rarely support a viable fishery. Restoration of fish habitat in such streams is not advisable unless there is a good possibility that adequate flows can be provided in the future.

The special conditions described above are exceptions. For most stream relocations installing special habitat measures is worthwhile and effective. Those that have proved reliable and practical are discussed in the following chapter.