OSL DATING OF HILLSLOPE SEDIMENTS IN SOUTHEASTERN WASHINGTON: AN ANALYSIS OF TOPOCLIMATE-MEDIATED HILLSLOPE EVOLUTION PATTERNS IN SPRING GULCH

Spring Gulch is a small tributary of the Columbia River located in Wallula Gap in Southeastern Washington. The landscape was heavily affected by the Missoula Floods, producing thick deposits of slack water sediments on valley floors and erosional scour features. Following the Missoula floods, aeolian reworking produced thick deposits of loess on hillslopes throughout the area. Spring Gulch is oriented east-west, producing hillslopes that are preferentially north- and south-facing, each with their own distinct topoclimates. The south-facing slopes receive more insolation leading to hotter conditions and increased evapotranspiration. South-facing slopes have more exposed bedrock, thinner soils, and less vegetation. Additionally, there are talus deposits, mixed with eolian sediment, at the bottom of the south-facing slopes indicating erosion of hillslope soils. The north-facing slopes receive less insolation and have cooler and effectively wetter conditions. There is less exposed bedrock, thicker soils, and denser vegetation. North-facing slopes have a consistently steep convex form with minimal talus deposits at the base of the slope, suggesting comparatively minimal erosion. We reconstructed a history of slope evolution using optically stimulated luminescence (OSL). Five samples were collected at depths ranging from 0.05m-1.63m from talus deposits on the south-facing hillslope. Four additional samples were collected at depths ranging from 0.07m-1.12m on the backslope of the north-facing hillslope. The south-facing talus slope accumulated sediment from 14-0.3 Ka at a rate of 0.1m/kyr. This suggests sustained erosion on south-facing slopes throughout the Holocene. The north-facing slopes accumulated eolian sediment on the back slope from 9.6-0.9 Ka at a rate of 0.1m/kyr. North-facing slopes show a net accumulation and thickening throughout the Holocene. The overall erosive environment on the south-facing hillslope is caused by a lack of vegetation and exposed bedrock which inhibits aeolian accumulation and promotes erosive overland flow. Aeolian accumulation on north-facing slopes may be enhanced by dense grass cover that traps sediment. These patterns may provide a model for understanding how the effects of anthropogenic warming will change hillslope ecosystems within a Wallula Gap area.