Bedrock rivers often alternate between relatively wide unconstrained reaches and conspicuously narrow deep incised bedrock reaches (canyons). These bedrock canyons exhibit a constriction-pool-widening (CPW) morphology that consists of a lateral constriction, a deeply scoured pool formed downstream of the constriction, and a channel widening at or near the pool exit. To explore how CPWs are formed in bedrock canyons, we hypothesize that the lateral constriction at the canyon entrance forces a CPW to form allogenically with subsequent CPWs propagating further downstream. Our hypothesis was tested experimentally in a flume channel with a forced lateral constriction at the canyon entrance. Our experiment shows that the forced constriction can cause a primary CPW to form allogenically because the backwater upstream of the forced constriction causes sediment deposition that creates an elevation drop, promoting flow and sediment to plunge toward the bed and carve a primary pool. Channel widening occurs at the primary pool exit because sediment deposit forms that deflects sediment into the banks, causing lateral erosion. Downstream of the primary widening, channel width declines and a new lateral constriction forms, which causes the formation of pools and widening downstream, resulting in downstream CPW propagation. In our experiment, the bedrock channel evolved until a persistent alluvial cover formed, reaching a steady state morphology without further vertical erosion until perturbed by higher discharge. Our experiment shows that discharge variation is necessary for a channel to evolve in the absence of uplift. Mountain rivers often alternate between wide, shallow channels that are not constrained by bedrock and deep, narrow canyons that are laterally constrained by bedrock. Canyons often exhibit repeating sequences of channel narrowing that are accompanied by deep pools and channel widening near the pool exit. Here, we conducted a laboratory experiment to better understand how these morphological sequences are formed. Our experimental bed and banks were made of a type of foam board commonly used for insulation because the foam board erodes like rock, but at a much faster rate. We setup our experimental channel so that there was an initial channel narrowing, like at the entrance to a canyon, and then studied the erosion patterns created by gravel transport through the simulated canyon. We found that sediment deposition upstream of the channel narrowing creates a steep water surface slope entering the canyon, which drives sediment into the bed carving a slot at the canyon entrance. The slot gradually widens forming a pool that is elongated downstream. At the pool exit, flow slows down and sediment deposition occurs, which then deflects sediment into the channel banks, creating a channel widening. This initial constriction-pool-widening is propagated downstream, creating new sequences of this morphology. Lateral constriction of bedrock canyons can cause the formation of pools through vertical erosion and the widening through lateral erosion The constriction, pool and widening morphology can self-propagate downstream Bedrock channel morphology can reach a steady state through a balance between uplift and lateral and vertical erosion rates
