Experiments on Pool Formation in Bedrock Canyons

Abstract

AbstractRivers cut into rock exhibit a wide range of morphologies that do not occur in alluvial rivers. The reach‐scale morphology of bedrock canyons has not been widely explored. Many rivers bound by rock on both banks exhibit a constriction‐pool‐widening morphology characterized by a lateral channel constriction, a deeply scoured pool that typically forms downstream of the constriction, and a channel widening coincident with the scour pool. Lateral constrictions are thought to cause plunging flows that carve pools which then shallow and widen downstream. A flume experiment was conducted to test this hypothesis. Experiments show that flow deceleration upstream of the constricted canyon promotes alluviation. Flow acceleration through the canyon prevents persistent alluvium from developing before a pool is formed. At the canyon entrance, flow and sediment plunge toward the bed, creating a primary scour pool. The primary scour pool reaches equilibrium morphology for a given constant discharge and sediment supply by cutting a slot, which then gets deep enough to maintain a permanent alluvial cover, protecting the pool from further vertical erosion. Downstream of the primary pool, an alluvial cover intermittently develops that causes flow to plunge, carving secondary pools. Shear stresses are counterintuitively large in alluviated areas and low in places where the bed is clear of sediment. However, near‐bed velocity was strongly correlated with alluviation patterns and erosion rate, suggesting near‐bed velocity may be a more practical way to calculate rock erosion rates in non‐uniform flows.