THE POTENTIAL EFFECTS OF FORECASTED CLIMATE CHANGE ON MASS WASTING SUSCEPTIBILITY IN THE NOOKSACK RIVER BASIN

The Nooksack River originates in the high relief regions of the North Cascades Mountains in northwest Washington State and drains an approximately 2300 km² watershed. The river is a valuable freshwater resource and provides critical habitat for endangered salmon species, as such it important to understand how sedimentation in the basin will respond to projected climate change. Previous climate change modeling studies in the basin indicate that the winter snowpack will decrease into the 21^st century and be restricted to regions above 1100-1300 m, about 500 m above historical averages. With about 60% more basin area snow-free and exposed to higher rainfall in the winter, runoff will increase and consequently raise the mass-wasting risk during the winter months and sediment delivery to streams. Our objective is to assess the increased risk by examining the high relief portions of the upper basin where steep slopes, young geology, and exposed glacial moraines contribute to high sediment yields.

As a first order assessment, we identify areas at risk of slope failure using an ArcGIS-based, non-probabilistic infinite-slope model along with digital soil and vegetation grids and a newly developed LiDAR coverage at multiple resolutions for the upper Nooksack basin. Regions at risk are further analyzed within the Distributed-Hydrology-Soil-Vegetation-Model (DHSVM) in conjunction with an infinite-slope failure model to determine the probability of shallow mass-wasting events from significant historical storm events. Comparing the winter historical snow-free area to the 2075 snow-free area, preliminary infinite slope modeling results reveal a 115% increase in landscapes that have a factor of safety less than 1.5, indicating a much higher risk of sediment delivery to streams. In order to project future sediment loads, historical turbidity, sediment, and discharge measurements will be examined with the goal of developing rating curves to be used in simulated streamflow outputs generated by future climate scenarios in the DHSVM.