COMPARISON OF LATE PLEISTOCENE AND HISTORICAL DEBRIS-FLOW VOLUMES AND INITIATION MECHANISMS, SANTA CATALINA MOUNTAINS, ARIZONA

On 31 July 2006, unprecedented rainfall caused hundreds of shallow (~1 m deep) slope failures in the Santa Catalina Mountains of southeastern Arizona resulting in debris flows that reached the apex of alluvial fans at the mountain front in five drainages. Using volume estimates of the debris flows, measurements of the debris-flow inundation, and the empirical debris-flow model LAHARZ, the extent of debris-flow runout from the 2006 event was modeled with reasonable success. This modeling success raises the possibility that the volume of prehistoric debris flows in the Santa Catalina Mountains can be estimated from the spatial extent of their runout deposits as determined by geologic mapping. The largest and most extensive debris-flow deposits are probably late Pleistocene, based on relative-age estimates corroborated with ¹⁰Be cosmogenic dating. On alluvial fans emanating from two canyons, debris-flow deposits were mapped up to 3 km from the mountain front; we assumed these deposits represent a single event for the purposes of volumetric modeling. We used LAHARZ to back calculate debris-flow volume using the observed runout distances and model coefficients derived from analysis of the 2006 event. The failure volumes necessary to create the late-Pleistocene deposits are on the order of magnitude of 10 million m³, or two orders of magnitude greater than the volumes estimated for the largest debris flows in the 2006 event. The uniform erosion of one meter of sediment from all hillslopes in these canyons would generate only half the sediment volume necessary to emplace the observed late-Pleistocene debris-flow deposits. These debris-flow volume calculations suggest that debris-flow initiation mechanisms associated with the present, late-Holocene climate conditions are insufficient to emplace these late-Pleistocene deposits. These data also suggest that climatic change between the late Pleistocene and the Holocene affected storm meteorology and seasonality thus causing changes in debris-flow initiation mechanisms, available source material, and initiation-source areas. These changes have implications for debris-flow hazard assessments.