Abstract: An apparatus constructable in situ for filtering pollutants from a flowing creek or stream, comprising one or more adjacent and/or cascaded flume-type boxes supported by the stream bed, each of which boxes comprises parallel sides, top and bottom spacers, at least one upwardly sloping ramp between the sides to form a weir edge, and one or more replaceable porous trays containing a filtering or pollutant-removing medium located directly beneath the weir edge. A slotted cover for the trays uniformly distributes the stream's contaminated water over and onto the filtering medium and a screen bottom for the trays retains the filter medium therein. The apparatus is positioned in the stream so that all the water in the stream will flow into the box or boxes, up the ramp, over the weir edge or edges, and downwardly through the filter trays before resuming its course along the stream bed.