Publication Date

Spring 2013

Degree Type

Thesis

Degree Name

Master of Science (MS)

Department

Geology

Advisor

Manny Gabet

Keywords

curvature, GIS, hillslopes, knickpoints, LiDAR, soils

Subject Areas

Geomorphology

Abstract

Base level lowering often leads to the migration of knickpoints up the fluvial network as the channel profile adjusts to the new lower boundary condition. In steep terrain, the passage of a knickpoint can oversteepen valley walls and trigger a wave of erosion up the hillslopes. As soil is stripped from hillslopes, the previously diffusive hillslopes are transformed to landslide-dominated. Soils in diffusive landscapes are well developed until erosion exposes the underlying saprolite by shortening the soil residence times. During base level adjustments, the erosion of hillslopes can leave relict patches of the original landscape juxtaposed with the newly evolving landscape. Recent incision in central Idaho has produced large channel-to-ridge relief along the Salmon River and has resulted in the propagation of large knickpoints into many of its tributaries. These knickpoints mark the boundaries between pre-uplifted terrain (relict landscapes) and freshly eroded terrain (refreshed landscapes). In this study I aimed to analyze the hillslope response to the passage of a knickpoint by comparing morphological characteristics between relict and refreshed landscapes. A transect situated on both relict and refreshed landscapes was established to measure soil properties and ridgecrest morphology. The spatial analysis used the National Elevation Datatset (NED) and high resolution Light Detection and Ranging (LiDAR) elevation data. Soil analysis showed 1) higher percentage of gravel in the refreshed landscape, 2) a higher percentage of carbon in the relict landscape, and 3) similar average soil depths in both landscape types (∼18 cm). Spatial analysis showed the mean slope angle in the relict landscape is 18±7° and 33±7° in the refreshed landscape. Prospect Ridge has three distinct values of curvature: 0.0033±0.001 (relict), 0.0219±0.008 (refreshed), and 0.0668±0.009 (close to Salmon River). Three sets of increasing relative levels of erosion were therefore inferred from these curvature values. The erosion rate corresponding to the refreshed landscape is responsible for the formation of the large knickpoints within the site. Hillslopes downstream of the large knickpoints are subject to rapid oversteepening and complete landscape transformation from diffusive to landslide-dominated. Conversely, a less dramatic hillslope response was observed upstream of the knickpoint based on evidence of slight channel lowering, partially oversteepened valley walls, and pockets of hillslope steepening in tributary sub-basins. Reconstruction of relict hillslopes and the pre-incisional relict channel suggest that a much smaller, extinct knickpoint preceded the larger knickpoint and rapidly diffused into the headwaters.

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