Two white Walling tubes floating on the surface of a river

In order to identify the source of suspended sediment in the stream, long-term, aggregated samples are gathered by channeling water through Walling tubes, the white tubes shown here.  The water that flows through the tube is slowed and the sediment drops out; the water is able to exit from the other side of the tube.  The sampling technique is complementary to "grab" samples from the stream-it samples suspended sediment from all parts of a stream, but provides no information on the abundance of sediment in the stream.