HILLSLOPE RESPONSE TO KNICKPOINT MIGRATION: AN INVESTIGATION OF SLOPE STABILITY IN THE CULLASAJA RIVER BASIN

Processes responsible for base level fall within fluvial networks include eustasy, surface uplift, stream capture, and differential stream incision due to either lithologically controlled variations in rock mass strength or stream power contrasts at tributary junctures. A fall of base level leaves the local landscape in disequilibrium. Landscapes respond to base level perturbations via the upstream migration of knickpoints through trunk and tributary channels, and should force concomitant responses from adjacent hillslopes. Knickpoints represent a transient portion of the landscape separating the actively adjusting downstream reaches from upstream portions that preserve the relict landscape. Many workers have investigated knickpoint migration through trunk channels, tributary networks, and the response of hillslopes to knickpoint migration; however, the linkage between hillslope response to base level fall and knickpoint migration remains poorly understood. To this end we investigated the Cullasaja River basin located in the southern Blue Ridge Mountains of western North Carolina. The Cullasaja basin is ideal for a study of this kind because: 1) its main channel contains multiple large knickpoints; 2) it is tectonically inactive, and consists of a relatively consistent lithology – both attributes that simplify the search for linkages between knickpoint migration and hillslope evolution; and 3) high-resolution (6 m) LiDAR DEMs are available for the entire basin, permitting detailed GIS analyses. Our hypothesis is that knickpoint migration from the Cullasaja trunk through tributary networks locally steepens hillslopes, forcing adjustment of the landscape, and as a result slope failures are more likely to occur on these hillslopes. To test this hypothesis we use the combination of GIS, Matlab, and Stream Profiler tools (geomorphtools.org) analysis software suite to extract long profiles, identify knickpoints, and analyze a variety of stream and hillslope metrics. Our findings in combination with those from the North Carolina shallow slope stability mapping project (SINmap), suggest that knickpoint migration critically steepens hillslopes making them more susceptible to slope failures.