Influence of catchment lithology on debris‐flow fan morphology, sedimentology and evolution—Field evidence from the White Mountains, southern California, USA

Debris‐flow fans form by repeated deposition of debris‐flow sediments. Catchment lithology affects debris‐flow grain‐size distribution, and thereby rheology, erosive potential and depositional morphology. We can therefore expect that lithology also influences debris‐flow fan characteristics. Here, we determine how catchment lithology affects the surface morphology and sedimentology of debris‐flow fans and by extension their spatiotemporal evolution. We study nine fans along the eastern margin of northern Owens Valley, California, USA, originating from catchments with contrasting lithologies, and similar climate, tectonics and geological history. Results show that debris flows originating from catchments comprising magmatic rocks are rich in cobble‐ to boulder‐sized grains. The coarse sediment along the flow fronts and margins minimizes lateral spreading of debris‐flow lobes, forming distinct levees and thick depositional mounds. In contrast, debris flows originating from catchments dominated by sedimentary rocks are rich in relatively fine gravel. Their fine‐grained levees and lobes lack strongly frictional margins, spread more easily and form distinctly thinner and wider deposits. Debris flows originating from catchments with metamorphic lithologies show intermediate grain size and depositional morphology. These contrasts in debris‐flow characteristics guide the morphology and spatiotemporal development of debris‐flow fans. Fine‐grained debris flows spread laterally and tend to fill topographical lows, whereas lateral spreading of coarser‐grained flows is hampered, instigating a low tendency to fill topographic lows. The more efficient topographic compensation on fans formed by fine‐grained debris flows causes smaller elevation differences across a less rugged surface and is likely to lead to higher avulsion frequencies. The limited mobility and spreading of coarse‐grained debris flows promote frequent deposition on top of and directly adjacent to channel margins, forming well‐defined channels bordered by thick composite levees and raised fan sectors. These results illustrate how catchment lithology can affect the morphology, sedimentology and evolution of debris‐flow fans, providing guidelines for reading their depositional archives and avulsion hazard assessment.