Meredith, C. P. Budy, and J Schmidt. 2018. Investigating scour depths in relation to patterns of spawning brown trout and the changing physical template of a mountain river. Submitted to River Research and Applications, 28 December, 2017. USGS FSP:
In the Intermountain West, USA, eggs and fry of fall-spawning brown trout are susceptible to scour-related mortality because they are still in the gravel during spring snowmelt run-off events. The goal of our research was to understand patterns of gravel scour on the Logan River, Utah in relation to brown trout spawning preferences and whether mobility of spawning gravels could explain the absence of brown trout from higher elevations. We collected data to characterize local and reach-scale potential for spawning gravel scour (i.e., “entrainment potential”) at representative reaches along a longitudinal gradient during 2009 and 2010 spring flood events. We also used scour chains to measure scour depth at spawning locations. The flood magnitude in both years approximated the 2-year flood magnitude, but the flood in 2009 was much longer in duration. Scour at 27% of scour chain locations exceeded our estimated median depth of developing fry in 2009, while scour at 0% of locations exceeded this depth in 2010. Fewer low-gradient reaches were present in the upstream, non-canyon sections. However, across all reaches, we found that brown trout preferentially chose locations in channel margins for spawning, which tended to have low entrainment potential. These local estimates of entrainment potential predicted scour depth better than reach-scale estimates or broad-scale patterns of geomorphology. Some of the greatest scour depths were measured in the backwaters of a dam, a location with low entrainment potential but potentially greater sediment supply. Our findings suggest that spawning gravel scour is not limiting brown trout abundance at high elevations in this system given shallow scour depths overall and selection of low-scour areas for spawning.