Silt deposits at river mouths form landmasses called deltas, which are essentially small areas of wetlands. Deltas form when rivers empty their water and sedimentary materials into secondary bodies of water such as oceans, lakes and other rivers. Deltas may also empty on land, although this occurs much less frequently.
Deltas form in various time periods, although the largest deltas take many years to accumulate. The process begins as rivers move more slowly upon reaching their end points. The flow of river water gradually lessens as it approaches the mouth, or end, of the stream. As a result of a slowing pace of movement, sediment and debris have a chance to accumulate. The combination of reduced water velocity and accumulation of sediment creates deltas, which contain a variety of different organic substances, including salt, dirt, sand and minerals.
Occasionally, these deposits form deltaic lobes, which are series of distributary networks, or branches of smaller and narrower channels, which branch off the mainstream of rivers. These smaller networks contain heavier and coarser materials at their front ends, while lighter materials deposit at their rears. Deltas may be singular or divided into two parts, called subaqueous and subaerial. Subaqueous deltas are underwater, while subaerial deltas are above.