Wildlife and Habitat

Restoration of Fish Habitat in Relocated Streams

FLOW IN NATURAL STREAMS

Rivers and streams are nature's channels for the disposal of surface runoff. Stream flows vary continuously from day to day, month to month, and year to year, and the variations between the greatest floods and the lowest flows can be very large. A ratio of 100 to 1 is not uncommon.

A stream forms its channel by erosion, practically all of which takes place during a few days or weeks in the year when the stream is in flood. The main channel-forming flood for most streams is the discharge that has a 50 percent chance of being equaled or exceeded in any given year. The larger floods which occur less frequently may considerably alter the work of the 50 percent chance floods, but after they are over, the stream will resume the normal channel forming process. It is this process that is most significant for creating and maintaining fish habitat.

During floods of any size, a stream may carry large amounts of sediment suspended in the flow. Most of this material is clay, silt and fine sand washed into the stream with the surface runoff, but a small amount is derived from the stream's own bed and banks. It is this bed and bank material that is most important for creating and modifying fish habitat.

Action of a Stream on Its Bed and Banks

Gravity, acting on the water and its suspended sediment, is the propelling force of stream flow. This force is opposed by the friction exerted on the flow by the bed and banks of the stream. As the volume flowing in a stream increases, the stream's erosive power also increases. The resulting erosion varies with the resistance of the materials composing the bed and banks, from practically nil for bedrock and large boulders to very extensive for sand and silt.

During high flows, lighter materials, such as sand or fine gravel are swept up by the turbulence and carried along in suspension in the flow. Heavier particles such as coarse gravel may roll and slide along the bottom. When the flood recedes, the heavier particles stop rolling and lighter ones drop out as the velocity decreases. The bed of the stream is changed after each flood. Pools and gravel bars may remain in substantially the same places, but the bed materials have changed and the stream is in fact slightly different.

High flows sweep away sand and small gravel on the stream bed, leaving the large gravel, cobbles and boulders that are too heavy for the current to move.* A pavement or armor is formed which protects the undisturbed materials below from scour. As the flood recedes, this armor is covered by finer materials, but it remains to protect the bed from the next flood. If the armor is removed, as for example by gravel mining, a new cycle of erosion can begin, and the streambed will "degrade" until a new armor surface develops. Lack of armor explains why new relocated channels can cut themselves deeper than the original excavated depth although the new channel may have the same or a flatter gradient.

*Sediment particles are commonly classified according to their diameters as:

After a stream has armored its channel, its bed may be immune to erosion from all but the highest flows. However, the stream may still scour the unarmored deposits in the banks, providing continuing supplies of sediment in the channel forming process.

With time, the steep upper portions of a stream tend to degrade their beds while the lower portions build up wide flood plains of sediment. Eventually, the stream reaches an equilibrium where the gradient at any given place, the flood flows and the sediment load are in balance. When this occurs the stream is barely able to carry away the sediment that comes to it from upstream, and it is neither cutting or building up its bed. This balanced condition can be upset, by natural or manmade changes in the stream's geometry, sediment load or discharge. If, for example, the channel is shortened by relocation, the local gradient will be steepened, which may cause the stream to begin cutting its bed above the relocation. The sediment thus created will be deposited downstream, building up the bed. Any significant change of the natural channel has the potential to start a new cycle of erosion, and must be considered in the hydraulic design of every stream relocation.

Obstructions to Flow Cause Changes in Stream Channels

An immovable obstruction, such as a large boulder, restricts the area of flow in a stream channel. Water piles up against the upstream edge of the obstruction, causing an increase in velocity around the sides, accompanied by the development of vortexes which scour the bed. Scour holes may develop, and the scoured-out material may be deposited downstream as a gravel bar.

If the obstruction is overtopped during high water an additional erosive force is introduced as water plunges over the downstream face, impinging on the bed like a jet. This force can greatly enlarge the scour hole below the obstacle.

Scour holes change somewhat with each flood, but as long as the obstacle remains the scour hole will persist unless filled by an excess of sediment in the stream.

An obstruction which extends completely across the channel, such as a low dam, has the effect of flattening the gradient upstream. This reduces the stream's velocity, and also its capacity to transport sediment. The stream may drop some of its sediment in the pool above a new dam and this pool may eventually become filled with deposits.

Downstream from the dam the opposite effect occurs. The water falls almost vertically from the lip of the dam, impinging on the bed very much as a jet from a hose. The strength of this jet increases with the volume of water passing over the dam, and the height of fall. If the bed materials are of such size that they can be moved by the jet a scour hole develops immediately below the dam. The smaller materials are scoured out first, followed by larger particles until only those remain which are too large to be moved by the water. The scour hole is then "paved" or "armored" and will not deepen itself any more until a greater flood occurs.

If the bed material below the dam consists of very coarse particles the scour hole may be very small or may not develop at all. In such cases, a hole or basin should be excavated in the bed when the dam is installed to provide the desired plunge pool for fish habitat.

Material removed from a natural scour hole is carried downstream where, as the velocity decreases it forms a gravel bar. Such bars are often excellent spawning beds for fish.

Scour holes have a tendency to enlarge themselves in all directions, so the foundation of a dam must either be built below the probable final scour level, or the scour must be controlled by placing coarse rock along the toe of the structure.

An obstruction at the edge of a stream constricts the flow and changes its direction toward the opposite bank. This can cause increased far bank erosion, and at the same time cause a scour hole to develop below the near bank tip of the obstruction. These "current deflectors," whether occurring accidentally in nature or introduced by man, are important molders of fish habitat in streams.

Obstructions which cause an appreciable constriction of the flood flow of a stream may cause sediment bars and riffles to form upstream. This tendency is intensified if the channel is unusually wide above the constriction. In nature, clusters of boulders, fallen trees or debris lodged in the channel may cause constrictions. Bridges and bank riprapping may also constrict the flow if not properly designed.

Channel Formation in Straight Streams

Natural streams are never absolutely straight, even where they have steep gradients.* They tend to develop bends as in Figure l(b). Erosion is greater in the outsides of these bends, and the eroded material is carried downstream, where it is deposited as bars in the insides of the bends. These bars force the stream against the opposite bank, causing more erosion and deepening of the channel. Eventually, the bends become pools and the gravel bars form "riffles," and the stream assumes an undulating profile as shown here.

The relationship between pools and riffles varies with stream discharge. During low flows, a stream is more sinuous, and riffles, pools and slow shallow water tend to increase. During high water the stream tends to straighten; riffles and pools tend to decrease, and deep fast runs tend to increase.

In straight streams having beds composed of several sizes of coarse materials, the riffle-pool sequence commonly occurs at intervals of 5 to 7 stream widths. On the average, pools tend to be longer than riffles, and the bed materials in riffles are noticeably coarser than in pools. For a stream that is in equilibrium, the gravel bars tend to remain in the same location for years, although the materials composing them may change from year to year.

*Geologists define a straight stream as one in which the length measured along the line of greatest channel depth is less than 1112 times the length of the valley in which the stream flows.

Riffle-pool sequences will not develop in streams with sand or silt beds.

Straight streams usually have hard, erosion resistant banks of gravels, boulders or even bedrock, which restrain the stream's natural tendency to develop a sinuous course. The gradient may change abruptly with bed conditions, producing short sections of swift water alternating with relatively flat reaches.

Meandering Streams and Braided Streams

Streams with comparatively flat gradients and erodible banks are more sinuous than straight streams. The bends tend to extend themselves laterally to form loops or "meanders" as in Figure l(a). If the bed materials are coarser than coarse sand or fine gravel, the riffle-pool sequence will develop in meandering streams, with pools in the outsides of the bends and riffles in the crossovers from one bend to the next. Frequently, pools develop overhanging banks providing excellent cover for fish.

Natural or man-made obstructions in a stream may cause or reinforce meandering in unwanted places. This possibility must be considered when designing and placing habitat improvement structures. Riprap bank protection may be needed at vulnerable places to prevent extension of meanders.

Streams carrying large quantities of sediment may develop a braided pattern as in Figure l(c). Such streams usually have steep gradients and carry more sediment than the stream can effectively transport. They tend to have wide, shallow beds when in flood, and to cut their channels even wider, producing more sediment to add to the excess already existing. These streams lack the riffle-pool sequence, and the channel may spread out in many shallow rills incapable of supporting fish.

In its course through the country, the same stream may be straight, meandering or braided, depending on the local topography and geology.