The Ecology of Glacial Rivers: The role of beavers

The fifth in a series from the Susitna Salmon Center: The Ecology of Glacial Rivers. This segment by Jeff Davis explains how beavers improve habitat for salmon.

Gravel bars that develop as glacial rivers move across the floodplain expose soils allowing for the establishment of wind-blown seeds of cottonwoods and willows. Willow stems transported and deposited on the gravel bars of glacial rivers also can develop roots and continue to grow in these new locations. Cottonwoods and willows along glacial rivers provide a food source for herbivorous beaver (Castor canadensis). While herbivorous, the Alaska Department of Fish and Game has published observations of beaver feeding on salmon carcasses in the Susitna River.

Beaver, like salmon, are considered a keystone species, a species that has a large effect on their environment relative to their abundance. Beaver are present in the main channels and off-channel habitats of glacial rivers. Beaver in the main channel construct lodges in the riverbanks but do not try to construct dams in larger rivers. Beaver can often girdle and topple large cottonwood trees with 2 or 3 foot diameter trunks. Downed cottonwood trees allow beaver to access small branches near the tree top throughout the winter. When these large trees fall into the river they create slow-water eddies used by migrating adult salmon and rearing juvenile fish.

Beaver construct dams in side sloughs and upland sloughs of glacial rivers altering conditions within these habitats. In some side sloughs, beaver dams may be under the water surface during high summer flows, but as water levels drop during the fall and winter beaver dams maintain water depths and provide overwintering habitat for juvenile salmon. Beaver dams in off-channel habitats also provide protection from high water velocities during floods. However, beaver dams in side sloughs may only be present for a year or two and are often blown out during flood events or spring breakup.

Beaver dams constructed in upland sloughs that are not connected to the main channel at the upstream end, tend to be much higher than those in side sloughs and can stay in place for decades. Leaves and organic matter accumulate in beaver ponds in upland sloughs that are not periodically flushed by main channel flows. The large amount of organic matter can make these productive rearing habitats for juvenile coho salmon. However, over time can result in acidic waters with low amounts of dissolved oxygen, particularly under winter ice cover, resulting in poor habitat for rearing salmon.

Beaver dams are rarely considered barriers to the movement of juvenile salmon that can easily migrate through or around the porous dams. Beaver dams in glacial rivers may be temporary barriers to adult salmon migrating to spawning habitat in side sloughs depending on the dam height and water depth below the dam. Adult salmon are often seen holding below a beaver dam waiting for fall storms to increase water levels to allow access across or around the beaver dam.

Beaver are common in glacial rivers and have evolved with salmon through the years. Studies have linked the loss of beaver to declining salmon populations and beaver are currently used in some situations to restore streams and improve fish habitat.

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The Ecology of Alaska’s Glacial Rivers, by Jeff Davis

Episode 5. Beavers in glacial rivers