Flume experiments were conducted to investigate the mechanisms of transport of a gravel-sand mixture by shallow unidirectional flows. Two water recirculating sediment feed flumes were used: one with a 6 m long and 0.15 m wide channel and other with an 11 m long channel with widths of 0.74 m and 0.53 m. The sediment, poorly sorted gravel with a mean size of 3 mm, was fed at the upstream end of the channel at steady rates from 0.03 kg s−1 m−1 to 1.0 kg s−1 m−1. Sediment transport rate out of the channel varied in all runs, at approximate periods of 3 min in the runs with high transport rates to 14 min in the runs with low transport rates. The runs with low transport rates also showed fluctuations in total transport rate at periods of about 25 min. Transport rates of each size fraction varied with time in a distinctive pattern in all runs. The time variations were caused by the migration of very long and low bed load sheets in the runs with low to moderate transport rates and dunelike bed forms in the run with the highest transport rate. The bed surface in all runs was coarser in size than the original sediment mix except that with the highest transport rate (run H5) in which the size distribution was nearly the same as the original.