Large scale depositional features are formed where stream velocity suddenly drops and a large quantity of sediments are suddenly left stranded. The resulting landforms grow larger with time.
Deltas
Where a stream reaches its base level (e.g., a lake or the ocean), there is no longer any gravitational energy to produce a current. Velocity drops to essentially zero and all of the sediments are deposited. Coarsest sediments are deposited first, near the mouth of the stream, progressively finer sediments are deposited farther out in progressively deeper water. As the sediment builds up near the mouth of the stream it blocks the stream's passage. The stream migrates to another location. Sediment builds up in the new location until it blocks the way. Then the stream finds yet another path. Eventually a network of distributary streams (the opposite of tributaries) distribute the water and sediment over a wide arc. The sediments also build outward into the lake or ocean. Finer sediments (fine sand and silt) are deposited in front of the expanding delta in the form of gently inclined foreset beds. The foreset beds are finally capped by sandy topset beds and more foresets are deposited in front of the growing delta. Fine-grained (silt and clay) bottomset beds are deposited beyond the foresets and eventually new foresets are deposited over them and topset beds over them.
On the delta, distributary channels alternate with freshwater and brackish marshes that periodically receive sediments when the river floods. Upstream damming of rivers, and especially, the building of artificial levees to control flood waters and constrain the coarse of the river prevents these sediments from being deposited on the delta lowlands and marshes. The loss of sediment influx combined with subsidence due to the removal of groundwater for growing cities (and also the weight of the delta sediments on the crust) and the slow steady rise of sea level means that parts of some deltas, like the Mississippi Delta, are actually being reclaimed by the sea.
Alluvial Fans
Where a stream tumbles out of a narrow rocky mountain valleys onto a flat valley, stream width increases and stream depth decreases while stream gradient also decreases. As a result, stream velocity suddenly drops and sediments are deposited. Alluvial fans are best known in dry climates in places where block fault mountain ranges alternate with downdropped valleys as in the Basin and Range region of the western U.S. Alluvial fans grow as fan-shaped landforms that come to a peak where the stream meets the mountain front. The main stream migrates from side to side across the fan as sediment builds up too much in one area it diverts to supply sediment to another. Often, many smaller streams also help to distribute sediments in all directions radiating from the central peak of the half-cone shaped landform. Sediments may also be carried down the slopes of the fan by sheet wash during intense rain. Mudflow deposits are also found, interlayered with the alluvial deposits. Streams that flow across fans are typically intermittent or ephemeral because the coarse sediments of the fan allow the stream draining the mountains to rapidly infiltrate the ground after leaving their bedrock channels in the mountains. Slopes are steepest (perhaps 10 to 15 degrees) at the top of the fan and gradually decrease until they merge with the flat valleys below. The coarsest sediments (gravel and sand) are found on the upper portion of the fan. Grain size decreases to silt and clay as the fan merges with the valley floor.
As alluvial fans grow, fans from adjacent mountain streams may begin to overlap. Eventually a bajada, an apron of alluvial sediments may develop along the margin of a mountain range as fans from many streams grow outward together.
Old well-worn mountain ranges often develop a pediment, a sloping prism of sediment lapping over the bedrock of a largely eroded mountain range. The cause of pediments is poorly understood though they may be produced by sheet wash of sediments eroded from the mountains.