Ecology of Glacial Rivers: Side Channels

The fourth in a series from the Susitna Salmon Center: The Ecology of Glacial Rivers. This segment by Jeff Davis tells how side channels and side sloughs provide important salmon spawning and rearing habitat.

The balance between the amount and timing of high flows and sediment transported from glaciers results in a number of different kinds of habitats in glacial rivers. Previously, we compared the fast main channel that contains a large amount of suspended sediment and clear-water upland sloughs that develop in abandoned channels cut off from the mainstem. Other habitat types common in glacial rivers include side channels and side sloughs. These two habitat types differ in the amount of water they receive from the main channel compared to groundwater sources.

Side channels always receive water from the main channel and, like the main channel, contain a large amount of suspended sediment. Side channels have 20 percent or less of the flow compared to the main channel and are separated from the main channel by islands that are longer than the channel width. Water velocities are lower in side channels and the velocities and depths do not increase as much as in the main channel during high flow events. Because there is not as much water flowing down these channels, and flows are not as variable, riparian bank vegetation such as willows and alders, extend to the water line. Undercut banks and overhanging riparian vegetation provide cover for rearing juvenile salmon and other fish. Because stream energy is lower, logs that are carried in the main channel during high flows are often transported into and create log jams in side channels. There also is not enough flow in side channels to move large cottonwoods that fall into the stream. Logs and log jams provide additional cover, create pools, and trap leaves that provide food for aquatic insects and fish. Tributaries and clear ground water have a greater influence on the water quality in side channels because of their smaller size compared to the main channel.

As the main channel continues to move across the floodplain, side channels receive less and less water from the main channel. Side sloughs are created when water from the main channel only flows into a side channel for a short time during high summer or fall flows or just often enough to keep an upstream connection open with the main channel. Even where there is no surface flow, water from the main channel still flows into side sloughs through the streambed. As main channel water flows through the streambed, sediments are filtered resulting in clear waterside sloughs. Clear water allows for more sunlight to reach the streambed, increasing algal growth. Even though side sloughs do not always receive surface flow from the main channel, the main channel still controls water depths and velocities in side sloughs. When flows in the main channel are high, there is more water flowing through the streambed and into side sloughs. Water from the main channel can backwater into the downstream end of side slough channel increasing water depths. Juvenile Chinook salmon, coho salmon, and sockeye salmon are found rearing and overwintering in side sloughs of glacial rivers. Rearing juvenile salmon can hide from predators within the turbid glacial backwater at the downstream ends of side sloughs while still taking advantage of these productive habitats.

Recent studies conducted by the Alaska Department of Fish and Game have documented the movement of radio-tagged chum salmon, sockeye salmon, and coho salmon to spawning habitat in side sloughs throughout the glacial Susitna, Talkeetna, and Chulitna Rivers. Water upwelling through the gravel of side sloughs attracts spawning salmon and provides a constant source of clean water to incubating salmon eggs. Ice formation on the main channel during winter may force additional water through the streambed and into side sloughs throughout the winter. Periodic flooding from the main channel during storms or during ice development help maintain a source of spawning gravel in these channels.

Side channels and side sloughs differ in the amount of water that comes from the main channel in surface flow compared to water that flows through the streambed. Side channels and side sloughs provide important salmon spawning and rearing habitat.


The Ecology of Alaska’s Glacial Rivers

Episode 4. Side Channels and Side Sloughs

By Jeff Davis

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