keeper hole

A hydraulic powerful enough to hold and recirculate a swimmer or boat indefinitely — the recirculating current strong enough that the water pouring in at the upstream end overpowers any attempt to escape. Keeper holes form below ledges, low-head dams, and pourover drops where the hydraulic is uniform across its width, offering no corner or edge to wash out of. Low-head dams are the most lethal keepers because they look harmless from upstream. A keeper is the river's trap, and it is patient.