Six channel-spanning boulder weirs with other associated structural configurations were constructed along mainstem of Mosby Creek in 2011 to improve fish habitat. The structures were studied to evaluate their influence on water temperatures and their stability against hydraulic forces across a range of boulder sizes in different configurations. Boulders were configured as weirs, clusters, duos, barbs, and individuals, with mean volumes 1.4, 0.45, 0.42, 0.83, and 0.63 m3, respectively. Bankfull widths ranged from 9.1 to 12.2 m, with stream gradients between 0.8 and 1.2%. After summer construction, 671 boulders were surveyed and then resurveyed following winter flows. There were five high flow events during the first winter, with two above the 2-year recurrence interval. The weirs captured approximately 1200 m3 of alluvium after the first winter. Only minor movement of boulders was observed, with only seven moving > 10 m, and all weirs were intact. Boulders of similar density (mass/volume) in these flow conditions should be stable once boulder volume exceeds 0.5 m3. Three noteworthy findings were seen in the temperature analysis. Lower absolute maxima temperatures were observed directly below weirs. There was no change upstream or downstream of weirs, signifying that thermal change is local. Thirdly, maximum minima were similar following alluvial deposition, indicating little effect of daylight heat gain. While created hyporheic zones may have little stream-wide influence, small areas of cool water are thermal refugia for fish, particularly in systems where current temperatures can stress salmonids. Boulder weirs show promise as a method that develops such habitats.
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Vol. 90 • No. 4