A concurrent backwash distribution apparatus for a filter (11) having a filter tank (13) and a plurality of underdrain laterals (24) includes a baffle (10) positioned in a common main conduit (12) to define an auxiliary flume (14) between the baffle (10) and the underdrain laterals (24). The auxiliary flume (14) is in fluid communication with the common main conduit (12) and the underdrain laterals (24). The baffle (10) has at least one flume liquid metering orifice (16) preferably located near the bottom of the baffle (10) so that it may be below a gas/liquid interface in the auxiliary flume (14) during concurrent gas/liquid backwash operations. A flow distribution plate (18) may be positioned between the auxiliary flume (14) and the underdrain laterals (24), the flow distribution plate (18) having at least one liquid metering orifice and at least one gas metering orifice. The baffle (10) may take one of many forms. The baffle (10) creates an adequate liquid pool in the common main conduit for low velocity flow of backwash liquid while at the same time providing uninhibited flow of backwash gas. Maldistribution is thereby controlled. Stand-pipes (35) may be used in place of the baffle (10). A method for concurrent gas/liquid backwash according to the invention is also disclosed. The invention may be applied to flat bottom flumes or insufficiently recessed flumes.