River formations are a type of landform created by the deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. Deltas are a common type of river formation, which are formed when a river splits into multiple branches and deposits sediment into a body of water, such as an ocean or lake. The sediment builds up over time, creating a triangular-shaped landmass at the mouth of the river.