<P> A river delta is a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower - moving or stagnant water . This occurs where a river enters an ocean, sea, estuary, lake, reservoir, or (more rarely) another river that cannot transport away the supplied sediment . The size and shape of a delta is controlled by the balance between watershed processes that supply sediment and receiving basin processes that redistribute, sequester, and export that sediment . The size, geometry, and location of the receiving basin also plays an important role in delta evolution . River deltas are important in human civilization, as they are major agricultural production centers and population centers . They can provide coastline defense and can impact drinking water supply . They are also ecologically important, with different species assemblages depending on their landscape position . </P> <P> River deltas form when a river carrying sediment reaches either (1) a body of water, such as a lake, ocean, or reservoir, (2) another river that cannot remove the sediment quickly enough to stop delta formation, or (3) an inland region where the water spreads out and deposits sediments . The tidal currents also cannot be too strong, as sediment would wash out into the water body faster than the river deposits it . The river must carry enough sediment to layer into deltas over time . The river's velocity decreases rapidly, causing it to deposit the majority, if not all, of its load . This alluvium builds up to form the river delta . 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 deposits . Over time, this single channel builds a deltaic lobe (such as the bird's - foot of the Mississippi or Ural river deltas), pushing its mouth 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 (see slope). </P>

What is a delta and how does it form