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Rockbed Wear At A Flow Convergence Zone in Fifteen Mile Creek, Niagara Peninsula, Ontario

Sculpted bedforms developed in rock channels must be considered within the context of temporally and spatially persistent flow dynamics. A shallow meandering channel (14 m long, 30‐50 cm wide, 3‐4 cm deep) with raised margins has been eroded into resistant Irondequoit Limestone, forming the streambed of Fifteen Mile Creek; Ontario. Potholes (<18 cm deep, <25 cm wide, <35 cm long) within the channel increase in size downstream. Subaerial solution of the limestone and fracture patterns within the Irondequoit Limestone are not responsible for this. Large dolostone blocks within the channel stabilize patterns of vorticity that form in the lee of the blocks during highflows when flow is about 1.3 m deep. Vortices migrating downstream along a major flow junction have eroded the distinctive channel. Small irregularities in the bed may enlarge to potholes by attracting vortices and undergoing erosional wear as they pass. The feature may have developed within several hundred to several thousand years, but likely developed in relation to flows with magnitudes approaching the Mean Annual Flood, or larger. No other examples are known in Fifteen Mile Creek, or adjacent systems, and it is likely that the large block in the center channel dropped from an overhang and has not subsequently moved.