QUANTIFYING COUPLING BETWEEN LANDSLIDES AND STREAM INCISION ABOVE THE CASCADIA SUBDUCTION ZONE, CLEARWATER RIVER BASIN, WASHINGTON

Mass wasting induced by basin-scale hillslope adjustment to stream incision is a central mechanism of mountain system evolution that is often observed, yet rarely quantified. Conceptually, a basin may adapt to topographic adjustments along its margins with a ridge-ward propagating front of accelerated erosion that locally steepens slopes. Our hypothesis follows that intensified landsliding along these fronts is a major mechanism that propagates erosion in steeplands.

In the Clearwater River basin of the Olympic Mountains, landslide scars cluster along the lower hillslopes below a network of stream knickpoints. We hypothesize that pulses of stream incision initiated a wave of erosion that is now expressed as increased landslide frequency on hillslopes, and as knickpoints on streams. Prior investigations have revealed both tectonic and climatic forcing upon the topography of this catchment, which provides constraints to model hillslope response to stream incision. Reduced sediment supply or increased stream discharge during interglacials is responsible for incision and preservation of terraces, whose basal strath unconformities were formed during glacial periods. Incision into two terraces that formed in the late Pleistocene are the episodes suspected to have initiated stream incision and upstream knickpoint propagation. Using a suite of established modeling approaches, we will test if the timing of incision into these terraces, and the knickpoint migration that is modeled, is coincident with present-day knickpoint location.

Investigating hillslope-stream coupling will permit comparison of local landslide erosion and knickpoint migration rates. In the future, landslide erosion rates will be determined with multi-temporal landslide inventories, and knickpoint formation will be approximated by genetic divergence of fish populations above knickpoints. Here, we compare results of modeled knickpoint retreat and hillslope steeping given constraints provided by prior investigations.