LIDAR HELPS REVEAL THE LITHOLOGIC CONTROL ON BEDROCK RIVER SINUOSITY IN THE SANTA CRUZ MOUNTAINS, USA

The possibility that landscapes encode information about climatic and tectonic forcing motivates much of the field of geomorphology. Recent work highlighting correlations between river sinuosity and lithology, flood magnitude and flood frequency encourages detailed mechanistic exploration of the controls on river plan form morphology. Here, we take advantage of an excellent natural experiment in the Santa Cruz Mountains of California, where 1 meter LiDAR data are available for the adjacent Pescadero and Butano watersheds. These streams have similar climatic and tectonic histories, and comparable drainage areas, however the two channels have differing lithology (mudstone versus sandstone), sinuosity (~1.8 versus ~1.2), and terrace formation histories.

Bedrock erosion can be addressed as either a ‘boundary stress’ or a ‘boundary strength’ problem. Much of the current literature focuses on the erosive stress, or power, rivers can apply on their boundary. Climate, tectonics, and rock type all factor into this erosive potential. The variability in this potential, however, is orders of magnitude less than the variability in rock resistance to erosion. Because most actively meandering bedrock rivers cut through weak sedimentary rocks, we hypothesize that lithology is the main control on the development of sinuosity in the channels in our field area. More specifically, we hypothesize that Pescadero Creek is sinuous because it can migrate laterally and hence detach bedrock from the walls of the canyon it cuts through, whereas Butano is straight because its boundaries are essentially unerodible to clear water flows and hence stable to near-bank stress perturbations.

We tested this hypothesis by performing erosion measurements on these two rock types, including direct in-situ erosion measurements with a pressure washer device and laboratory slake durability measurements. The results of our in-situ experiments show that the Purisima formation is indeed much more detachable under clear water flows than the Butano. Additionally, our laboratory experiments show that production of detachable bedrock in the Purisima formation occurs due to seasonal wetting and drying of the mudstone.