Incision of steepland valleys by debris flows

Steepland valleys are prone to episodic debris flows, which are flowing mixtures of rock and water. Debate continues about whether debris flow valley incision is adequately represented by modified fluvial incision laws (e.g., stream power law) that predict power laws of drainage area against valley slope. Using a wide range of topography from debris flow-prone temperate steeplands in the U.S and around the world, I find that inverse fluvial power laws (straight lines on log-log plots) rarely extend to valley slopes greater than ∼0.03 to 0.10, values below which debris flows rarely travel. Instead, with decreasing drainage area the rate of increase in slope declines, leading to a curved relationship on a log-log plot of slope against drainage area. This curved relation is found along recent debris flow runouts in the U.S. with extensive evidence for bedrock lowering by the impact of large particles entrained in the debris flow, and along field-mapped runouts of older debris flows in the western U.S. and Taiwan. By contrast, downvalley from terminal debris flow deposits, where strath terraces often begin, area-slope data follow fluvial power laws. Valleys cut by debris flows have long-profiles different from those cut by rivers. To measure bedrock lowering rates in both places, I installed hundreds of erosion pins in the rock floors of steep valleys recently eroded by debris flows in Oregon, and in bedrock rivers in Washington, Oregon, California and Taiwan. I monitored these sites for 1–7 years. Pins in valleys scoured by debris flows have been buried by colluvium, indicating a lack of fluvial incision. By contrast, pins in riverbeds (with power law area-slope plots) have lowered at rates up to cm's per year, at values that are proportional to the square of bedrock tensile strength. Cultural artifacts in the fluvial deposits of adjoining strath terraces in Washington and Taiwan rivers indicate at least several decades of lowering at these extreme rates following historic exposure of bedrock. Observed lowering rates at most sites far exceed estimated long-term rock uplift rates, so the observed reaches of these rivers cannot be adjusted to either bedrock hardness or rock uplift rate in the manner predicted by the stream power law. Although power law plots of area versus slope may be consistent with regions of fluvial incision, they need not reflect the stream power bedrock river incision law. (Abstract shortened by UMI.)...