HABITAT RESTORATION MEASURES AND STRUCTURES

Over the years, effective and practical measures have been developed for improving fish habitat in streams. By applying these measures, game fish can be restored to relocated channels. Which measures to use will depend on the size and bed materials of the individual stream, and the available supply of suitable improvement materials.

The ideal in habitat improvement would be to establish a stable riffle-pool sequence in the relocated channel. Such a sequence will develop naturally if the stream carries enough coarse sediment. If insufficient sediment is available, the stream may need some help. For example, investigators in Scotland found that a straight relocated channel could be greatly improved by dumping loads of gravel into the stream about 5 to 7 stream widths apart. Over the next one or two seasons the annual floods leveled the piles and formed a stable riffle-pool sequence in the new channel. Where coarse gravel is plentiful and other materials such as large rocks are scarce, this can be an economical way to fashion habitat quickly in small streams.

The most commonly used habitat improvement measures are large rocks placed in the stream, current deflectors and check dams. The number, size and location of these will affect the stability and conveyance capacity of the new channel, and so should be considered concurrently with the hydraulic design.

Stream Improvement With Large Rocks

Large rocks are ideal construction materials for habitat improvements. If properly selected, they are durable and have a pleasing natural appearance which harmonizes with the river landscape. Glacial and river boulders, talus fragments, quarry stone or large pieces of rock from the roadway excavation may be used for stream improvement. Angular rocks are more effective than rounded ones. Since exposure conditions are severe, only rocks of known durability should be used.

The primary purpose of large rocks is to develop scour holes in the stream bed as resting places and cover for fish. If the bed consists of fine gravel or sand the scour may be great enough to undermine the rock and cause it to be buried in the bed. Rocks, therefore, should be used only where the bed material will be relatively stable during floods. Bottoms of large gravel or cobbles will be reasonably stable if flood velocities do not exceed 8 ft/sec (2.4 m/sec).

Large rocks may divert the stream current against one or both banks. If the banks are easily eroded, bank protection will be needed.

Habitat rocks must be large enough to resist displacement during floods. If the bottom is stable, a rock of 2 ft (0.6 m) diameter weighing about 1000 pounds (454 kg) will resist movement in current velocities up to 10 ft/sec (3 m/sec). A 4-foot (1.2 m) rock will be stable in velocities up to about 13 ft/sec (4 m/sec).

The maximum size should be related somewhat to the width of the stream. Usually, a rock should not be greater in its largest dimension than one fifth of the width of the channel at normal summer flows. However, for channels that are steeper than 3 percent some rocks may be as large as one third of the width to help dissipate some of the stream's energy. Table 1 is a guide for selecting sizes.

A rule of thumb is to allow one large rock per 300 sq.ft. (27 m2) of channel. These should be placed where the current is swiftest, which is usually in the center half of the channel in straight reaches, and the outside half in bends. This will produce the best scour holes, and also insure that the rocks will not be left on dry land during periods of low flow.

Generally, large rocks are effective only where the velocity exceeds 2 or 3 ft/sec (0.6 or 0.9 m/sec.) at medium flows. Rocks create better habitat when they project above the surface during low and medium flows, and when they are placed with the longest dimension perpendicular to the flow.

Rocks can be placed in the channel at random as in Figure 3 (c), or in systematic patterns as in Figure 3(a) and Figure 3(b). Diamond patterns can be used in straight or curved channels. In these patterns, the side rocks should be placed in the main jets of flow diverted by the middle rocks.

Large rocks may be placed in groups or clusters to create cascades and "stair-step pools" in steep streams. Such arrangements are effective for creating fish habitat, and also for reducing velocities by increasing channel roughness.

Where large rocks are not available, a substitute can be made of smaller rocks and cobbles enclosed in galvanized wire mesh to form "rock sausages", "rock baskets" or "gabions". These are esthetically inferior to large rocks, and their effective life is shorter due to failure of the wire mesh from abrasion and corrosion.

Current Deflectors

Current deflectors create large scour holes and pools by concentrating the flow of the stream in a relatively narrow part of the channel. The resulting pools are often longer than the stream width and deep enough to provide high quality fish habitat. Alternate deflectors in a straight channel can divert the flow into a meandering pattern. Double deflectors, placed opposite each other as in Figure 4. can cause a long and deep hole to form downstream.

The best materials for deflectors are medium-to large-sized rocks, preferably angular in shape. These do not have to be as large as the free-standing habitat rocks described previously, since they are interlocked to some extent in the structure. The rocks can be either placed or dumped, but the larger rocks should be arranged near the point of the deflector. To prevent undermining and failure, the body of the deflector should be embedded at least 2 ft (0.6 m) below the stream bottom, and it should extend into the bank about 10 ft (3 m) beyond low water level. The bank should be riprapped for 15 to 20 ft. (4.5 to 6.0 m) upstream and downstream from the structure to prevent undermining and washout.

Figures 5 and 6 show two types of rock deflectors

Deflectors can be made from wire-enclosed gabions as shown in Figure 7. Gabion deflectors have some disadvantages aside from their unnatural appearance. The upstream edge is subject to abrasion from the gravel bed load carried by the stream during floods. This can cut the wire enclosure, and cause the gabion to collapse. Floods will then disperse the cobble filling. Also, mesh that is not permanently submerged may eventually fail from corrosion. Nevertheless, an effective life of at least 10 years may be expected from gabion deflectors. As with other types, gabion deflectors should be embedded in the bed and banks of the stream and protected by riprap.

Logs and sawed timber are excellent materials for deflectors if they can be kept submerged. However they decay if exposed to alternate wetting and drying, and for this reason it may be more economical in the long run to use treated poles and timber. A simple log design is shown in Figure 8. Figure 9 shows a more robust design suitable for larger streams. The structure shown in Figure 10 combines a deflector with overhead hiding cover for fish. It is important that logs and timber be securely tied together in the structure by bolts or rebars.

Common dimensions, orientation and spacing of deflectors are shown in Figure 11.

Deflectors should not obstruct flood flows, and therefore they should not project more than 1.5 ft (0.45 m) above the stream bed. All deflectors direct the flow toward the opposite bank at low stream stages. When submerged during high stages their effect may be different, depending on the deflector design. Long, thin designs angled to the current, as shown in Figure 12(a) will direct flood flows toward the near bank, so the bank protection should be extended farther downstream for these types. Blunt, wedge-shaped designs such as the one shown in Figure 12(b), tend to shunt the flood flow away from the near bank and form a scour hole downstream near the opposite bank.

Check Dams

Check dams are low structures built across a stream perpendicular to the flow. In hydraulic engineering, the most common use for check dams is to decrease the slope and velocity of a stream to control erosion. The number of dams needed for this purpose depends on the energy head that must be dissipated in vertical drops to maintain the original hydraulic gradient of the stream at flood stage. The plunge pool below a check dam provides excellent fish habitat, and the downstream gravel bar often associated with the dam makes an excellent spawning bed. When used to enhance fish habitat, check dams should be placed far enough apart to insure that the pool below a dam is above the backwater of the next dam downstream. This will reduce the possibility that the habitat pool of the upper dam may fill with deposits.

A check dam should not block the annual spawning migration. Salmon and steelhead can negotiate single vertical jumps of 2 or 3 ft (0.6 or 0.9 m). However, the normal limit for adult trout is about 1 to 1.5 ft (0.3 to 0.45 m) and the drop at a dam should not exceed this amount. The depth of the pool below the dam should be 1 ½ to 2 times the height of the jump with a minimum of 2 ft (0.6 m). To insure this depth it may be necessary to excavate the pool at the time the dam is installed. The downstream pool should be at least 10 ft (3 m) long.

Degradation of the channel downstream from a check dam can lower the downstream pool and increase the drop to the point where the dam becomes a barrier during low stages of flow. To prevent this, it may be necessary to armor the channel for a short distance below the downstream pool.

Check dams can be made from large rocks, gabions, logs and timber or concrete. For stability, these structures should be embedded in the streambed at least 2 ft (0.6 m) and the ends should extend into the banks about 10 ft (3 m) beyond the low water line. Riprap bank protection is essential to prevent washout.

Check dams of large rocks can be used in streams of any size. For streams up to 20 ft (6 m) wide, the main rocks should weigh about 800 to 1000 lbs (360 to 450 kg). Rocks of one metric ton or larger should be used in streams of 30 ft (9 m) to 100 ft (30 m) width. Figure 13 shows a typical design for a rock check dam. The principal fault of this design is that at low flows the water may divide into a number of small streams passing over the crest, none of which is large enough for fish to navigate. The dam then becomes a barrier to migration. The split rock check dam shown in Figure 14 is intended to solve this problem by directing the low water flow into a narrow curved channel. During high flows this design has the same effect as a straight rock dam.

Logs and timber are excellent materials for check dams. Round logs have a natural appearance that harmonizes with the stream. A notch in the crest will concentrate low water flows for passage of migrating fish. Under conditions of seasonal wetting and drying such as would exist in a natural stream, check dams of untreated logs will last about 20 years, depending on the species of wood, and conditions of exposure. A much longer life can be expected from treated poles and timber. Treated poles are commonly available in sizes up to about 18 inches (455 mm) butt diameter. Log structures should be firmly tied together with bolts or rebars set in drilled holes.

Figure 15 is a log dam design that is practical for streams up to about 30 ft (9 m) width.

The Log V-Dam shown in Figure 16 can be used in streams up to about 50 ft (15 m) wide.

The Log and Plank dam, of Figure 17 can be used in streams less than 30 ft (9 m) wide. Preferably, the upper log and timber deck should be of treated timber.

Gabions can also be used for check dams, as shown in Figure 18.

Replacing Hiding Cover

The best hiding cover evolves gradually as the channel assumes its mature shape and the bank vegetation develops with age. However, some cover can be supplied quickly by placing large rocks in the channel as described earlier.