Fluvially sculpted rock bedforms in Twenty Mile Creek, Niagara Peninsula, Ontario

Sculpted rock bedforms (s-forms) on Whirlpool Sandstone are described from the bed of Twenty Mile Creek. The morphologies identified (undulating surfaces, rises, furrows, comma forms, spindles, potholes, and transverse troughs) are those described for sculpted bedrock forms (p-forms, s-forms) developed in subglacial environments. Additional morphologies identified are undercut downsteps and quarried surfaces in the lee of bedrock rises. Morphological differences between the subglacial and fluvial forms are attributed to the difference between confined conduit flow in the subglacial case and open-channel flow in the fluvial examples. In fluvial systems the descending bedrock surface presents rock differently to oncoming flows and favours certain s-forms (quarried lee faces and undulating surfaces). In a strong-flow fluvial environment s-forms develop by wear. Dominant sediment-transport modes are large clasts (up to metre dimensions) and suspended silt–clay with a small (< 7%) hard or heavy-mineral component. Hydraulic quarrying, which removes fracture-delimited blocks from the bed, interrupts the process. Although fluvial s-forms are similar to subglacial s-forms, large stable vortices may not exist in the fluvial context over the range of effective stages. Upper regime flow is common over bedrock reaches, and flow acceleration at small downsteps (knickpoints) magnifies the duration and spatial extent of wear effective velocities.