Individual distributaries of the Okavango Delta are subject to progressive blockage and ultimately are abandoned, on a time scale of 50-100 years, and new channel systems form in consequence. This constant migration of channels is apparently the result of sediment accumulation in channels which causes aggradation of channel beds, accompanied by vertical aggradation of the flanking vegetation. This aggradation ultimately reduces hydraulic gradients down the channels, resulting in a decline in flow velocity and hence progressive vegetation blockage. Water is re-directed as a result and new channel systems are created.