Topographic Controls on the Distribution and Timing of Spring Runoff in a Snow- Dominated Basin in Central Idaho

The anticipated warming of the Intermountain-West will most strongly affect catchments whose run-off regimes are at the cusp between snow and rain dominated. In order to understand the function of at-risk snow dominated catchments, we have instrumented Big Creek, a 1445 km2 tributary to the Middle Fork Salmon River in central Idaho with a mainstem stream gage and ten pressure loggers in tributaries that range between 6 km2 and 243 km2. For the ten tributaries and the mainstem, stage discharge relations were established over the course of a field season, resulting in three months of continuous discharge data that record both the spring runoff and multiple rainfall events. Because the watershed drains from West to East and encompasses 2000m of relief, Big Creek's North and South facing tributaries provide an ideal field site to determine the role of aspect, drainage area and hypsometry on the relative magnitude and timing of runoff. Initial analysis of the 2008 data suggests that in low elevation, snow dominated catchments, aspect plays the most important role in snow accumulation and retention, thus determing the magnitude and timing of spring run-off. This suggests that the consequences of warming climate will be most pronounced in north facing catchments where both biotic and geomorphic systems adapted to discrete, large discharge spring events will have to adjust to more gradual, lower volume runoff events.