CHANNEL REFILL PROCESSES FOLLOWING POST-FIRE DEBRIS FLOWS, PINALENO MOUNTAINS, ARIZONA, USA

Within the Coronado National Forest of southeastern Arizona, fire severity has increased substantially, and high severity fire recurrence intervals have decreased, particularly in our research area on the Pinalen͂o Mountains. The increase in fire severity can be attributed to historic fire suppression and changes in climate. The increase in fire severity has also altered the landscape response to precipitation; the decrease in soil cohesion and infiltration capacity following high-severity wildfires leads to increased surface runoff, which can initiate debris flows through increased surface erosion. As a result, the study region has experienced upwards of 15 fire related debris flows within the last 20 years. Given the high number of recent post-fire debris flows, it is imperative to understand the processes acting to refill the scoured channels with sediment since sediment availability influences debris flow recurrence intervals. Our research aims to understand the processes refilling the Wet Canyon channel with sediment following substantial post-fire-debris-flow scour in 2017. For our study, we performed repeat structure-from-motion (SfM) surveys in Wet Canyon to determine the dominant processes acting on the channel during summer and winter precipitation events. SfM results indicate that channel-focused processes are the dominant driver of channel change during the summer. This resulted in downstream transportation of localized sediment pulses during flood flow. It is hypothesized that channel-focused processes are dominant in the summer due to substantial overland flow from intense, localized, convective storms. Conversely, hillslope processes are the primary driver of channel change in the winter. This has resulted in hillslope failures on scoured hillslope toes at the channel margins, leading to an increase in sediment delivered to the channel. Hillslope processes are hypothesized to be dominant in the winter due to deep infiltration of snow melt, prompting failures of scoured hillslope toes. The preliminary results from this study suggest that hillslope processes could play a critical role in refilling channels with sediment following debris-flow scour.