UNDERSTANDING DEBRIS FLOW RESPONSE TO TRANSIENT UPLIFT VIA REORGANIZATION OF GEOMORPHIC PROCESS REGIMES AT THE MENDOCINO TRIPLE JUNCTION, CALIFORNIA, USA

Landscape responses to uplift have been widely documented, showing that fluvial channels steepen, and ridgetops narrow with increasing uplift rate. Previous studies have focused on understanding how the hillslopes and fluvial channels respond to an uplift pulse, but little is known about how the debris flow geomorphic process regime responds to transient uplift. This study is aimed at quantifying the spatial response of geomorphic process regimes in coastal catchments that track the northward-migrating pulse of uplift associated with the Mendocino Triple junction (MTJ), California. The demarcation of the study area into distinct geomorphic process regimes using the slope area analysis unraveled the changes in process regimes as the landscape responds to uplift. Three process regimes in the analyzed basins include the hillslope, debris flow and fluvial regimes. The areal percentage of hillslope, debris flow and fluvial regime ranged from 78.04% - 64.88%, 34.27% - 20.09% and 0.49% - 2.03%, respectively. The spatial extent of the debris flow regime in the basins was significantly (with 95% confidence) positively correlated with uplift rate (r = 0.79). The spatial extent of hillslope and fluvial process regimes showed a significant negative correlation with uplift rate (r = -0.75 and -0.65). These findings suggests that an uplift pulse triggers the spatial reorganization of dominant geomorphic processes in a way that encourages the expansion of debris flow scoured channels as the landscape increases in relief and adjusts toward steady state. Debris flows are therefore an increasingly important link between hillslope and fluvial processes as they are a crucial conduit through which uplift signal is transmitted to hillslopes and therefore have implications on how fast hillslopes adjust to steady state.