The Influence of Climate Change and Fire on Sediment Transport and Aquatic Habitat: a Case Study of the South Fork of Salmon River Basin, Idaho

In the western United States, climate change is predicted to increase fire severity and frequency, decrease snowpack, and alter the timing and volume of runoff. Wildfire often increases hillslope erosion rates, which alter the timing, quantity and grain-size distribution of sediment supplied to streams. In addition, extreme flood events may become more frequent with climate change and will impact the channel morphology, grain-size distributions and aquatic habitat. To understand the influence of climatically induced alterations in flow hydrographs and hillslope erosion rates, a one-dimensional bedload transport model is used to calculate sediment and flow routing through the South Fork of the Salmon River. The model includes a channel network with drainage areas between 1.5 to 125 square kilometers. To understand the impact of increased hillslope erosion rates, we varied the amount and grain size distribution of supplied sediment between model runs while holding the flow hydrograph constant (rain vs. snow dominated). We also use data collected from an extreme flood event in June 2010, which was the highest recorded flow on record, as a proxy for the impact of climate change. We measured the grain size distributions, and channel dimensions in seventeen channels before and after this event. These parameters only changed significantly in channels with large sediment inputs from landslides. We also model the following two scenarios: 1) pre-event channel and sediment supply conditions combined with a snowmelt dominated hydrograph and 2) post-event channel and sediment supply conditions combined with the extreme event hydrograph. We compare the predicted channel slopes, grain size distributions, flow hydraulics (e.g. flow velocity, shear stress) and bedload fluxes between each model run and throughout the channel network. Such calculations are used to better understand the impact of sediment supply and flow hydrographs on channel stability, and aquatic habitat suitability (e.g. grain size, flow hydraulics) for a number of threatened and endangered species in Idaho.