Identifying and Quantifying Sources of Fall Chinook Salmon Spawning Gravel to the Snake River in Hells Canyon

Abstract:

The Snake River in Hells Canyon supports a growing population of spawning Fall Chinook Salmon (Oncorhynchus tshawytscha) immediately downstream of the Hells Canyon Complex (HCC) of hydroelectric dams for the last 60 years. The long-term survival of this salmon run depends on the input of spawning gravel (25-150 mm) from local tributaries balancing the losses of spawning gravel through attrition and export out of the reach between the HCC and the Salmon River confluence. We are working to quantify the gravel input of these local tributaries at different time-scales and put this into the context of historical supply and transport.

Long-term total sediment production rates of these tributaries estimated through various methods have varied by over 2 orders of magnitude, but we have recently completed ¹⁰Be work to constrain these estimates. We are measuring the change in storage of Fall Chinook spawning-size gravel through repeat multibeam echosounder surveys of the riverbed. The limited amount of repeat data collected to date has shown complex patterns of change in the riverbed. One possible driver of this complexity is the episodic and spatially variable nature of sediment inputs from these tributaries. We are attempting to quantify the frequency of the debris flows or floods capable of transporting spawning gravel through digitizing historic imagery of the last 60 years to determine the recurrence interval. We are measuring the magnitude of these events by surveying tributary fans pre and post-event to measure the sediment volume and particle size produced by specific events. These floods and debris flows are driven by extreme rainfall or snowmelt events, so we have also reconstructed historical meteorological conditions to identify the triggering conditions for transport, and identify the areas where snowmelt or rainfall is the more likely trigger. We are currently testing whether the unique bedrock geology of Hells Canyon can be used as a tracer to identify the source areas and downstream transport of sediment in the canyon. While the HCC has trapped gravel transported from the Snake River immediately upstream, this input has been quite low due to particle attrition across long transport distances and low transport capacity of the reach just upstream of the HCC.