Rehabilitation of bedrock stream channels: The effects of boulder weir placement on aquatic habitat and biota

The placement of boulder weirs is a popular method to improve fish habitat, though little is known about the effectiveness of these structures at increasing fish and biota abundance. We examined the effectiveness of boulder weir placement by comparing physical habitat, chemical and biotic metrics in 13 paired treatment (boulder weir placement) and control reaches in seven southwest Oregon watersheds in the summer of 2002 and 2003. Pool area, the number of boulders, total large woody debris (LWD) and LWD forming pools were all significantly higher in treatment than control reaches (p < 0.05). No differences in water chemistry (total N, total P, dissolved organic carbon) or macroinvertebrate metrics (richness, total abundance, benthic index of biotic integrity etc.) were detected. Abundance of juvenile coho salmon (Oncorhynchuskisutch) and trout (O. mykiss and O. clarki) were higher in treatment than control reaches (p < 0.05), while dace (Rhinichthys spp.; p < 0.09) were more abundant in control reaches and no significant difference was detected for young-of-year trout (p > 0.20). Both coho salmon and trout response to boulder weir placement were positively correlated with difference in pool area; p < 0.10), while dace and young-of-year trout response to boulder weir placement were negatively correlated with difference in LWD (p < 0.05). The placement of boulder weirs appears to be an effective technique for increasing local abundance of species that prefer pools (juvenile coho and trout > 100 mm). Based on our results and previous studies on bedrock and incised channels, we suggest that the placement of boulder structures is a useful first step in attempting to restore these types of stream channels. Published in 2006 by John Wiley & Sons, Ltd.