Biological and water quality responses to hydrologic disturbances in third‐order forested streams

We compared stream ecosystem responses to two types of disturbances: flood and debris flows. A large storm in February 1996 disturbed four similarly sized sub-watersheds of the Calapooia River, in the Cascade Mountains of Oregon, USA. All sub-watersheds had flood and in two, 31 and 81% of the perennial channel, a debris flow affected the channel. For 8 years, we used a suite of approaches: stream temperature, nutrient regime, periphyton and macroinvertebrate assemblages, and resident trout abundance and habitat to evaluate the persistence of instream impacts. Differences in stream temperatures and nitrate-nitrogen concentrations were evident, probably, due to the removal of vegetation and modification of riparian soils at the debris flow sites. Instream biological responses varied. After the event, fish, including trout, were rare with no fish at the debris flow sites. Within 6 years, trout densities (Oncorhynchus mykiss and Oncorhynchus clarki) were similar and young-of-the-year trouts were common. In contrast, periphyton and macroinvertebrate assemblages differed. Periphyton biomass was lower and nitrogen-fixing periphyton was more abundant at the debris flow sites. Macroinvertebrate assemblage diversity was higher at the debris flow sites due to fewer dominant taxa. Macroinvertebrate functional feeding groups also differed with fewer gatherers and more scrapers at the debris flow sites. Debris flow impacts related to loss of riparian canopy will probably persist until mature red alder stands are re-established along stream-reaches affected by debris flows to provide nitrogen input and shade. Copyright © 2011 John Wiley & Sons, Ltd.