Flow in vascular beds where lateral pressure tending to collapse vessels greatly exceeds venous pressure. Flow is independent of venous pressure and occurs only when arterial pressure exceeds lateral pressure; likened to the flow that forms a waterfall from a sluice or dam spillway, with arterial pressure being height of water behind the dam, lateral pressure being spillway height, and venous pressure being height of outflow stream below the dam. SYN: sluice.