Vertical Distribution of Sediment and Organic Debris in Coho Salmon (Oncorhynchus kisutch) Redds in Three Small Oregon Streams

Abstract

Vertical distribution of sediment (particle diameter < 3.33 mm) and organic debris was studied in coho salmon (Oncorhynchus kisutch) redds in three Oregon coastal streams subjected to different degrees of logging. Gravel composition in frozen cores varied significantly among redds in both logged streams. Sediment content in redds appeared to decrease with depth in the logged streams, but differences in gravel composition among streams obscured differences among depths. Sediment layers of variable thickness were visible in several core samples. In one logged stream, redds contained an average of 25% less sediment than redds used 1 yr earlier. Organic content of the gravel represented < 2.8% of the core samples by weight and was directly related to the quantity of sediment in the streambed. The pattern of vertical distribution of organic debris in the clearcut stream (most abundant near the surface) differed significantly from that in the other two streams. Patterns of distribution and variability in gravel composition were consistent with post-logging changes in gravel permeability and survival to emergence of coho salmon.