Abstract: A waterslide has a controllable weir operable to increase the release of water from an upstream pool upon sensing an approaching rider, and is particularly arranged for riders on flotation devices coupled into arrays. The weir has vertically pivotable damming structure across a local peak in the sluice at the exit point of the pool. Photosensors detect approaching riders and signal a controller to open the weir momentarily to pass the rider and a gush of water, by operation of a pneumatic linkage. Between operations a continuous flow of water increases the depth in the pool. The weir can be high enough to block passage of riders when raised, allowing timed release.