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Bedload transport exhibits considerable spatial and temporal variability, as reflected by large fluctuations of transport rates. Among the various mechanisms proposed for explaining this variability, bedform migration is often cited as the main cause. We took a closer look at this issue by running long-duration experiments in a gravel bed flume using constant water discharge and sediment feed rates. We monitored bed evolution and measured bedload transport rates at the flume outlet using high-resolution techniques. The bed was initially flat, but within a few hours bedforms consisting of alternate bars and pools had developed. The bars exhibited a stick-slip motion: they were stable for long periods but moved episodically (every 10 hr on average). Their downstream migration produced 36% of the sediment volume transported, mostly in the form of intense pulses. Much of the transport was caused by the displacement of sediment waves from pool to pool.

Plain Language Summary Sediment transport rates show considerable variability with time, even if the water discharge is constant. It has long been realized that rivers develop bed forms such as bars and dunes, which can migrate upward or downward. Their slow motion has thus been viewed as the main cause of fluctuations of sediment transport rates. However, streambeds free of bed forms also experience highly fluctuating sediment transport. We took a closer look at this issue by running a long-duration experiment using a tilted flume filled with gravel. The water discharge and sediment feed rate were kept constant at the flume inlet. Using recent high-resolution techniques, we monitored bed topography and sediment transport rates at the flume outlet. Quickly after the experiment started, a continuous series of bars and pools formed and occupied the entire flume length. In contrast with earlier observations, we noted that these bars mostly stayed at the same place. Every 10 hr on average, they moved downstream, producing bursts of sediment transport. In our experiment, the main source of fluctuations for the sediment transport rates was the cyclic erosion and filling experienced by pools.