UC Santa Cruz

Tectonically active orogens typically produce deeply incised river valleys.These deep river valleys create steep valley walls which increase the likelihood of slope failure. As landslide deposits fill valleys, rivers can either mobilize the deposits or incise new valleys around the landslide toes. Here, I investigate Arroyo Hondo, a river in central California’s actively uplifting Diablo Range, a part of the Pacific Coast Ranges of Northern California. The Oak Ridge earthflow is a slow-moving landslide that has been failing into Arroyo Hondo for at least the last century, translating blocks of Franciscan Mélange down into the steep-walled valley below. Through remote sensing, field measurements, and hydraulic modeling, I show that Arroyo Hondo has steepened its profile due to boulder input at the landslide toe so that it is capable of mobilizing the largest grain size fraction (D99) of the deposit. These results show that landslide-impacted river reaches, despite receiving extremely coarse material from hillslopes, can evolve to move the coarse sediment supply, like other alluvial rivers. This point is underscored by the fact that river slopes have relaxed below inactive landslides in the field area, suggesting diffusion over time from boulder transport.