River deltas form when a river carrying sediment reaches a body of standing water, (e.g , lakes , oceans or reservoirs ). When the flow enters the standing water, it is no longer confined to its channel and expands in width. This flow expansion results in a decrease in the flow velocity, which diminishes the ability of the flow to transport sediment.

As a result, sediment drops out of the flow and are deposited to form a platform of alluvium. Over time, this single channel will build a deltaic (a triangular tract of sediment deposited at the mouth of the river) lobe , pushing its mouth further into the standing water. As the deltaic lobe advances, the gradient of the river channel becomes lower because the river channel is longer but has the same change in elevation.

As the slope of the river channel decreases, it becomes unstable. Water under the force of gravity will tend to flow down slope. If the river could breach its natural levees ( a ridge of sediment deposited naturally alongside a river by overflowing water), it splits into different channels called distributaries. The distributaries build up their own levees. Also, as the gradient of the slope becomes gentle, speed decreases which will result in deposition of sediment within the channel and for the channel bed to rise relative to the floodplain. This will make it easier for river to breach it levees , over time , a mature delta will gain a distributary network.