Quantifying fine sediment infiltration in spawning gravel used by Chinook salmon (Oncorhynchus tshawytscha) in the Sauk River Basin, Washington, 2018–21

Abstract

Fine sediment can infiltrate into river substrate that salmonid fish species ( Oncorhynchus spp.) use to spawn. High levels of sediment infiltration can increase egg-to-fry mortality, which corresponds to the period when salmonids are still residing in the subsurface gravels. This study quantifies fine sediment infiltration of Chinook salmon ( Oncorhynchus tshawytscha ) spawning habitat during the egg-to-fry emergence period over three years in the Sauk River, which has naturally high fine sediment loads and important native salmon populations. Additionally, this study qualitatively assesses how grain size distribution of the riverbed and adjacent gravel bars compare to grain size distribution following fine sediment infiltration to evaluate if riverbed or gravel bar grain size distributions may provide information on the potential for fine sediment infiltration in spawning gravels. Fine sediment infiltration into spawning gravels was quantified using sediment boxes and infiltration bags that were installed in artificial redds constructed at known Chinook salmon spawning locations at three study sites on the Sauk River over the expected egg-to-fry period. Over the three-year study period (August 2018–April 2021), fraction finer of sediment (grain sizes of less than two millimeters), ranged from 0.12 to 0.23 across the three study sites and years. Based on a comparison of field observations from this study and percentage egg-to-fry survival curves found in the literature, the expected survival for Chinook salmon eggs in the Sauk River is roughly 30 percent. Expected survival increases to approximately 90 percent if eggs are eyed and thus farther along in their development. Our field study did not evaluate the progression of infiltration, so it is unknown if observed fine sediment infiltration was at this relatively high rate during the period that corresponded to early egg development, when eggs are more sensitive to fine sediment infiltration. Dissolved oxygen in the gravels is largely above critically low levels (4 milligrams per liter) during sensitive periods corresponding to egg development and is interpreted not to affect egg-to-fry mortality. Active channel morphology in the middle reaches of the Sauk River may pose an additional challenge to pre-emergence survival. Channel change, deposition, and potential scour at the middle Sauk River study site likely contributed to low recovery rates of both sediment boxes and infiltration bags in two of the three study years. In terms of grain size distributions of riverbed sediment and adjacent gravel bars representing the potential for fine sediment infiltration into spawning gravels, both riverbed and gravel bar bulk subsurface sediment samples had higher fraction finer for the representative fine grain size of two millimeters compared to the sediment boxes and infiltration bags. Therefore, riverbed and gravel bar samples may serve as a conservative first order proxy for potential fine sediment infiltration into spawning gravels, with the understanding that these samples may overestimate fine sediment infiltration by up to 15 percent.