Long-term effects of catastrophic debris flows on stream ecosystems of the Klamath Mountains, Northern California

Large rainstorms can trigger debris flows in headwater drainage basins that travel far down channel networks, scouring riparian vegetation and reworking valley floor deposits. We examined the long-term effects of debris flows on channel characteristics and aquatic food webs in steep (5%-9% slope), small (4-6 m wide) streams by comparing 5 streams that had recent debris flows in 1997, 3 streams that had debris flows in 1974, and 2 streams that have not been disturbed in the past 80+ years. Debris-flows decreased channel complexity by reducing alluvial step frequency and large woody debris volumes. Coarse benthic organic matter was present at very low levels in streams with recent debris flows. Recently scoured streams were dominated by dense, young white alder (Alnus rhombiflia), while unscoured streams had diverse riparian assemblages. Benthic invertebrate shredders, especially stoneflies, were more abundant and diverse in unscoured streams, reflecting the more diverse allochthonous resources. Periphyton biomass was higher in unscoured streams, but primary production was greater in the recently scoured streams, suggesting that invertebrate grazers kept algal assemblages at an early successional state in the recently scoured streams. Glossosomatid caddisflies were the predominant grazer in recently scoured streams; heptageniid mayflies were abundant in unscoured streams. Rainbow trout (Oncorhynchus mykiss) juvenile abundances in streams that had recent debris flows were generally similar to abundances in undisturbed streams. Summer stream temperatures were 2-3 degrees C higher in streams that had recent debris flows, which could reduce thermal refugia for threatened populations of salmonids in the mainstem Klamath River. Debris flows have significant impacts on stream communities, primarily through the cascading effects of riparian vegetation removal, that last for decades. Because debris flow frequency increases following road construction and timber harvest, the long-term biological effects of debris flows on localized and downstream aquatic resources have management implications.