Although a discordance in bed elevation between mainstream and tributary channels is a common feature of most river confluences, this height differential is largely ignored in many models of junction flow dynamics. Here we present new laboratory flume data that demonstrate the significant influence of bed discordance on flow at an acute angled confluence. A difference in bed height between mainstream and tributary channels is shown to obliterate flow deflection at the bed and create a distortion of the mixing layer between the flows, resulting in fluid upwelling at the downstream junction corner. This upwelling is responsible for the absence of a flow separation zone near the bed at the downstream junction corner and the lack of a zone of marked flow acceleration in the postconfluence channel. These experiments reveal complex flow fields generated by bed discordance and suggest the need for major revisions to current models of river channel confluence flow dynamics and their application to sediment routing at these sites.

Despite the widespread use of stream restoration structures to improve fish habitat, few quantitative studies have evaluated their effectiveness. This study uses a meta-analysis approach to test the effectiveness of five types of in-stream restoration structures (weirs, deflectors, cover structures, boulder placement, and large woody debris) on both salmonid abundance and physical habitat characteristics. Compilation of data from 211 stream restoration projects showed a significant increase in pool area, average depth, large woody debris, and percent cover, as well as a decrease in riffle area, following the installation of in-stream structures. There was also a significant increase in salmonid density (mean effect size of 0.51, or 167%) and biomass (mean effect size of 0.48, or 162%) following the installation of structures. Large differences were observed between species, with rainbow trout ( Oncorhynchus mykiss ) showing the largest increases in density and biomass. This compilation highlights the potential of in-stream structures to create better habitat for and increase the abundance of salmonids, but the scarcity of long-term monitoring of the effectiveness of in-stream structures is problematic.