Climate change and spawning site suitability in low-gradient unconfined mountain streams: Mechanistic analyses supported by an aquatic-terrestrial lidar

Background/Question/Methods

A climate change scenario commonly proposed for higher-elevation watersheds in the northern Rocky Mountains is a shift from current snow-melt dominated annual hydrology to systems with more rain-on-snow and higher flows during winter months. We explored whether such potential increases in winter flows could mobilize the gravel bed in critical spawning streams in central Idaho and damage incubating eggs of autumn spawning steelhead and Chinook salmon. We also predicted sand mobility as a function of flow rate in these systems where increased winter sand transport could clog redds. Sediment mobility forecasts were made with a two-dimensional fluid dynamics model supported by high-resolution channel topography mapped by the Experimental Advanced Airborne Research Lidar (EAARL). The spatially-explicit model predictions were calibrated against field measurements of water elevation and velocity and bedload transport.

Results/Conclusions

In an unconfined gravel-bed channel with a median grain size of 35 mm and a gradient of about 0.15%, model results and field observations suggest that gravel mobility remains very low, even at the most extreme bankfull flows. This indicates that extant gravels are likely a legacy of the glacial history of the area. Gravel mobility increased considerably in a second stream with a median grain size of 24 mm and gradient of about 0.3%. Enhanced sand mobility over the gravel bed in these streams appears to be a greater threat to redds than does bed scour. Prior studies of the effects of forecasted climate change on rivers, including the attributes of channel hydrology, morphology, hydraulics, sediment transport and aquatic physical habitat, were limited by localized or poor resolution information about stream topography. The unique EAARL bathymetric lidar data define the channel boundary conditions necessary for mechanistic flow and sediment transport models that support more powerful investigations of the geomorphic domains within which climate change may affect channel physical characteristics and habitat.