Abstract: A hydraulically operated set of riffles includes a network of spaced-apart tubular riffle bars arranged to lie above a collecting surface in a riffle box. A plurality of generally downwardly directed apertures is formed in each riffle bar, and a flange extends generally upwardly along the length of the riffle bar. An inlet of each riffle bar is connected to a source of water or other fluid. In operation, water or other fluid that is supplied to the riffle bars passes forcibly outwardly through the apertures and is subsequently constrained to flow outwardly through the spaces between adjacent riffle bars to prevent undesired accumulation of particles between adjacent riffle bars.