- The effects of beaver (Castor canadensis) on stream morphology, riparian zones, and coho salmon (Oncorhynchus kisutch) fry in Cape and Cummins Creeks, Oregon, were examined using stream surveys, vegetation transects, and coho salmon fry counts in 1987. The basin around Cape Creek has been extensively logged since the late 1940's. Cummins Creek is surrounded by primarily old-growth forest.
Cape Creek had higher densities of coho fry (P < 0.0004) than Cummins Creek. Coho fry were two to ten times more numerous in pools (P < 0.001) than in glides or riffles, respectively. The density of beaver dams in the autumn on the two streams was 1.2 and 1.1 dams/km on Cummins and Cape Creeks, respectively. By building dams, beaver added 7% and 14% more pool habitat in Cummins and Cape Creeks, respectively. This extra pool habitat was used by coho fry. Beaver ponds were larger than non-beaver ponds and thus contained more coho fry per pond. Densities of coho fry were, however, the same between beaver and non-beaver ponds. Beaver preferred to cut salmonberry (Rubus spectabilis), alder (Alnus rubra), and vine maple (Acer
circinatum) stems 2-9 cm in diameter for food and dam construction. Mean distance from the stream for foraging was 17.5 m (SDS=10.1). Beaver foraged up to 90 in from the stream bank along tributaries. The percentage of
available stems (for salmonberry, alder, and vine maple) cut for the small size class (0.5-3 cm) decreased after 5 in from the stream, after 20 in for the medium size class
(3-9 cm) and remained stable out to 30 m for the large size class ( > 9 cm). Most cutting tended to be upstream of the dam site. The mean area affected by cutting around the dam site was 1944 sq. in (SD=921 sq. m). Proximity to a logjam, tributary, or debris slide; midstory conifer cover; overstory conifer cover; and vine maple cover were factors that separated used beaver sites from unused random sites using stepwise discriminant analysis. Proximity to a logjam, tributary, or debris slide was positively associated with the discriminant function and accounted for 50% of the variability in the discriminant function. Vine maple cover was also positively associated with dam location in the discriminant function, while midstory and overstory conifer covers were negatively associated with the dam location. Based on the results of this study, I believe beaver
provide extra coho salmon rearing habitat, especially in late summer during reduced water flows. In heavily managed streams, beaver may be able to provide structure and
stability in the stream at a much lower cost than stream rehabilitation projects.