ASSESSING HOW ROCK TYPE AND RELATIVE EROSION RATE CONTROL THE CONCENTRATION OF BEDROCK-INCISING GULLIES; GABILAN MESA, CENTRAL COAST RANGES, CALIFORNIA

Gully formation is often associated with severe land degradation, posing local and global environmental problems. Here we assess the role that underlying rock type and erosion rates play in determining the concentration of bedrock-incising gullies in the Gabilan Mesa, in the central Coast Ranges of California. Specifically, we propose that the concentration of gullies is greater within mudstones than within neighboring sandstones and conglomerates. Mudstones of the Pancho Rico Formation break down during wetting and drying, through a process known as slaking, which we propose encourages the headward erosion of gullies. We mapped the concentration of bedrock-incising gullies over a 700 km² area using 0.3 -m -resolution satellite data, or 1 -m -resolution elevation data. We quantified the density of gullies by calculating the total length of gullies in a given area, using both a 3 km diameter moving window and the area of 77 manually selected drainage basins. Measures of gully density within drainage basins show increased concentrations in basins that are predominantly underlain by the mud-rich Pancho Rico Formation. This is highlighted by maps of gully density measured with a moving window, which show abrupt increases in gully density across Pancho Rico Formation lithologic contacts. We expand on these results by investigating controls on gully density within a particular rock type. While direct measurements of erosion rates are unavailable, we use the depth of incision into the initially horizontal (now tilted) Paso Robles Formation surface that characterizes the mesa top of this region as a proxy for relative erosion rate. While the concentration of gullies increases when moving from low to moderate relative erosion rate, this trend does not persist through the highest fraction of uplift and erosion rates. We speculate this may arise due to the difficulty of mapping gullies in satellite imagery from the highest relative erosion rate regions, which tend to be more heavily vegetated. These findings indicate that variations in underlying lithology and perhaps erosional history can trigger variations in erosional processes.