COMPARISON OF CHANNEL GEOMETRIES IN TRIBUTARIES LOCATED ABOVE AND BELOW A KNICKPOINT ON THE SOUTH FORK EEL RIVER, NORTHERN CALIFORNIA

A base-level lowering along the South Fork Eel River (SFER), California, is recorded by a prominent step (knickpoint) in the long profile, ~135 km upstream from the mouth. Long profile analysis and previous studies show knickpoints in tributary basins of the SFER, suggesting knickpoint propagation is a dominant process of base-level lowering and channel incision in the SFER. We utilize data from field measurements, USGS 10m digital elevation models (DEMs), and LiDAR-derived DEMs to compare channel geometries of bedrock-dominated tributary basins located upstream and downstream of the SFER knickpoint.

Field measurements of valley and channel widths in Standley and Bear Pen Creeks, (downstream of the SFER knickpoint) are compared to LiDAR-derived measurements in Elder Creek, (upstream from the knickpoint). Preliminary data show a power-law relation indicating narrower widths in Elder Creek; the relation for valley width against drainage area for tributaries upstream and downstream of the SFER knickpoint yields exponents (b’) of ~0.4 and ~0.3, respectively. A power-law relation of slope against contributing drainage area, measured from LiDAR data, yields basin-wide concavity value (θ) of ~0.43 and steepness index (k_s) of ~46 for the upstream basin. Preliminary analyses of slope-drainage area plots of tributaries downstream from the SFER knickpoint (extracted from USGS 10 m DEMs) display significant scatter and do not demonstrate strong power-law relation.

Using a reference θ of 0.45 we compare k_s values between study basins. Data suggest Elder Creek is steeper when scaled to contributing drainage area. Thus, results do not indicate downstream tributaries are narrower and steeper due to a base-level drop initiated by a propagating SFER knickpoint, as may have been expected. In fact, preliminary analyses indicate that right bank tributaries to the SFER (such as Elder Creek) are generally steeper than left bank tributaries. Other factors, such as differences in lithology, may contribute to varying steepness indices. Knickpoints are present in all study basins; however, knickpoints in Elder Creek are unrelated to migration of the current SFER knickpoint, ~15 km downstream. Our results suggest channel geometries in the study basins do not detect the signal associated with recent knickpoint propagation of the mainstem SFER knickpoint.