Experiments on constriction-pool-widening channel morphology in bedrock canyons

Bedrock rivers often alternate between relatively wide alluvial reaches and conspicuously narrow, deeply incised bedrock reaches (canyons) where there exists a constriction-pool-widening (CPW) morphology that consists of deep scour pools that form downstream of where the river is constricted laterally within a canyon, then widens downstream of the pool. I hypothesize that CPW morphology is forced by initial constriction of the channel by bedrock, but that subsequent CPWs within a canyon are autogenic. This hypothesis was tested experimentally in a flume channel, modelled after the Fraser Canyons, British Columbia, with bed and banks made from erodible foam simulating bedrock that is eroded by transported gravel. My results show the first CPW in a series emerges due to structural controls on river width, forming a pool at the entrance to the canyon that widens downstream. The results demonstrate that once formed, a CPW can then self-propagate allowing for the autogenic formation of additional CPWs downstream.