PREDICTING BEDFORM STABILITY IN STEEP GRAVEL-BEDDED STREAMS

Steep mountain streams are often characterized by the presence of bedforms, which have been observed in numerous field studies to transition from alternate bar (pool-riffle) to step-pool to cascade morphology with increasing channel bed slope. Different bed states can lead to additional roughness and flow resistance not present in low-gradient streams, which greatly affects sediment transport and bedrock incision rates in steep channels. However, the mechanics behind what controls bed states in mountain channels remains unclear. Flume and field studies have suggested that alternate bar sequences form under certain channel width-to-depth ratios; step-pools form when the flow is near supercritical conditions or when the channel width is narrow compared to the grain size on the bed; and cascade morphology may be related to debris flow initiation or transport. However, the connection between these controlling variables for bedform stability and channel bed slope is not clear. Here we combine previous work that identified process controls on bedform state with theory for hydraulics, sediment transport, and channel form in steep mountain streams to explain how and why each bed state is tied to certain channel slopes. We find that some bedform states require certain ranges in bed slope, consistent with observations. However, for other channel slopes, multiple bedform states are predicted to be stable, suggesting that a competition process may lead to the emergence of the most stable bedform. Although empirical evidence from field data supports a strong correlation between bedform type and channel-bed slope, we observed atypical bedforms for natural rivers in steep flume experiments, such as stepped alternate bars and upstream migrating alternate bars. This suggests that a clear separation of causation and correlation is needed for bedform stability to predict flow resistance, sediment fluxes and channel evolution under different land-use conditions, climates, or on other planets.