A river delta forms by the deposition of sediments from the river when it slows to join a larger body of water. Relatively fast-moving rivers can carry huge amounts of sediment eroded from their banks, with the speed of their flow keeping the sediments from settling to the river bottom. When the river joins an ocean or other body, the slowing of the water allows the sediment to settle.
Oceans in particular have waves to break down deposits of sediments, but some rivers carry enough sediment to accumulate it anyway. When many layers of sediment are deposited, they begin to affect the river's flow. Often, they block the river bed so much that it cannot carry the river's flow, and the river begins to branch out to multiple, smaller waterways. This creates the classic branched pattern of river deltas.
Deltas are important structures, housing both human and animal life, sometimes in abundance. The Nile river delta, for instance, supported the Egyptian civilization for thousands of years. For other organisms, this meeting of slow-moving salt and fresh water allows for remarkable diversity of life. Some organisms specialize in surviving in these unique environments, and many marine animals use deltas as temporary, relatively safe havens.