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.