...Streamside Forests Control Habitat-Damaging Sediments and Provide Organic Energy to Downstream Reaches

As stream channels get wider in a downstream direction, the widening partition between the streamside forest canopies allows sufficient light to promote the growth of benthic algae, especially diatoms. Many species of invertebrates known collectively as "grazers" specialize in eating diatoms and, in turn, provide important food to fish. In large rivers that are very wide and deep, planktonic algae can become the dominant food resource with forest litter being less important.

The downstream changes in channel size and shape and the organic food base along the river continuum greatly affect the fish population. For example, fish populations change from invertebrate-eating Salmonid fishes, such as trout and salmon, in the headwaters to plankton-feeding Cyprinids and Catostomids, such as carp and suckers, in large rivers.

A well-washed gravel stream bottom is necessary for Salmonid spawning beds - photo.

Photo right: A well-washed gravel stream bottom is necessary for Salmonid spawning beds.

Streamside forests facilitate the downstream flow of food by contributing large stable debris to the streambed. This stable debris is the mechanism by which the detritus is held long enough to be processed by the invertebrate community. Without debris dams, much of the organic input from streamside vegetation would be washed downstream without contributing to the life processes of the aquatic food chain. Only as the streamside forest nears maturity, is it able to produce organic debris in sufficient size and quantity to provide relatively stable detritus catchments.

The streamside forest helps to control sediment flux, the fourth factor affecting Salmonid survival, by stabilizing stream banks. Sediment concentrations must be very high (above 20,000 ppm) to cause mortality in adult fish by clogging the gill filaments and preventing normal water circulation and aeration of the blood. However, abrasion damage to gills begins to occur at sediment concentrations as low as 200 ppm.

In addition, low concentrations can cause behavioral changes and disrupt normal reproduction by covering spawning grounds and preventing the emergence of recently hatched fry. Sediment covering spawning grounds reduces the flow of intragravel water, limiting oxygen availability to incubating eggs and newly-hatched alevins and hindering removal of metabolic wastes. It similarly affects aquatic insects habitat, thus altering species composition of a major trout food source. Large instream debris can help store sediment, moderating transport rates and buffering against rapid changes in sediment loads that could cover spawning gravels, fill rearing pools and reduce invertebrate populations.


photo above: Well developed root systems help stabilize stream banks.		photo above: An example of a sediment-laden branch entering a less turbid stream.


photo above: Increased stream sediment concentrations cause behavioral changes in fish and can result in migration, reduced reproduction and/ or death of a species.		photo above: A juvenile Salmonid.

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photo above: Incubating eggs.

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