Potholes preserved along abandoned sections of bedrock-bounded rivers should provide significant paleohydraulic data if the conditions for pothole formation are understood. In a prototype study, we recognize two kinds of potholes along the paleo-Potomac River near Great Falls, Virginia. Vertical potholes, drilled into the strath floor, are circular in plan and are interpreted to form at sites of small stream depths and high flow velocity. Lateral potholes, noncircular with overhanging roofs, are found on flanks of rock obstacles. They are interpreted to be eroded by sediment-laden vertical flow near the air-water interface. The vortices circulate water in the horizontal plane, and begin to form near the free surface in flow separation zones downstream of the obstacles. The geometry of flow separation zones and thus the location of the lateral potholes are scaled by flow Reynolds number. Therefore, lateral potholes could be used to reconstruct paleoflow depths and velocities if parts of the channel bed adjacent to the flow obstruction are preserved.

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