Morphodynamic and Biotic Responses to Sediment Sluicing and Natural Floods in a Regulated Mountain River

Abstract:

Many regulated rivers now receive coarse sediment augmentation below hydraulic structures that interrupt natural alluvial transport. Because of limited accessibility, mountain rivers are often not amenable to sediment contouring using large machinery to create precise process-habitat feedbacks. One solution is to simplify the approach by focusing on water and sediment input requirements through direct intervention to obtain short-term habitat benefits that sustain existing populations, while also promoting long-term passive restoration driven by natural floods to recover historic population sizes. The question arises as to how introduced coarse sediment moves through a mountain river in response to floods and to what extent high-quality habitat is produced and used by fishes at injection locations and downstream where sediment deposits. In this study, gravel and cobble sluicing was designed, implemented and monitored from 2010-2014 with four injections of ~ 4,500 tonnes each into the Englebright Dam Reach of the lower Yuba River in semiarid north-central California. This is ~ 8% of the sediment deficit. A flood of ~ 7x bankfull discharge occurred after the first two injections. Morphodynamic change and habitat utilization were monitored and a bioverified 2D microhabitat model was used to evaluate the value of constructed and self-formed morphological units. Before the flood, 80% of habitat utilization observations occurred in spots predicted to be high-quality microhabitat. The flood redistributed sediment throughout the reach, exhibiting 5 mechanisms of topographic change. Downstream deposits formed high-quality habitat where it backed up against large bedrock outcrops and boulders, though much of the material simply filled in the moderately deep canyon; more of the sediment deficit will need to be met before widespread high-quality habitat will be available. Overall, sluicing is working to produce habitat through time in a challenging setting.