Decades of delta avulsion (i.e. channel abrupt jump) studies have not resolved what the main controls of delta avulsion are. Using a computer model, integrated with field observation, analytical, and laboratory-made deltas, we found that the sediment load, which itself is controlled by the steepness of the river upstream of a delta, controls the timing of avulsion. We can now better understand the main cause of abrupt channel changes in deltas, a finding that aids flood risk management in river deltas.