Long Profile Development of Bedrock Channels: Interaction of Weathering, Mass Wasting, Bed Erosion, and Sediment Transport

Erosion of bedrock channels seldom involves solely hydraulic processes such as plucking, abrasion, and solution. Weathering, mass wasting and burial by sediment cover modulate the rate of bedrock erosion. In headwater channels weathering generally must reduce rock strength to the point that entrainment may occur by hydraulic processes or rapid mass wasting. Simple quantitative models are introduced that demonstrate how erosion rates can depend upon both weathering rate and stress applied by moving fluids and debris. Rockfalls and avalanches can trigger additional failures in partially weathered bedrock on lower parts of alpine bedrock slopes before they would fail solely by weathering; this generates an economy of scale that results in development of spur and gully landforms. Streambed weathering also enhances bed erosion by water and debris flow in headwater bedrock channels within moderate relief landscapes. In large bedrock channels erosion rates are controlled both directly and indirectly by the throughflowing sediment. Abrasion by bedload and suspended load is often the dominant process. The rock beds of many streams are mantled partially or shallowly by alluvium. Two primary issues are unresolved about long-term evolution of these mixed bedrock-alluvial channels: 1) how and when the bedrock is eroded and 2) whether the gradient is determined by the necessity to transport the alluvium or to erode the bed. A semi-quantitative model suggests that bed erosion occurs due to exposure during extreme floods, at the base of migrating bedforms, and during periods of low sediment influx. Erosion rates in rapidly downcutting bedrock channel reaches are often regulated by influx of boulders that partially or wholly mantle the bed. These locally contributed boulders are primarily derived from steepening of sideslopes and tributaries due to the rapid incision.