Multiple terraces can sometimes be seen, resembling steps in a giant staircase. The abandoned floodplain, or stream terrace, can be seen well above the new stream channel elevation. The stream did not have time to erode its old floodplain by meandering over it, so it was preserved. Abandonment occurred when the erosive power of the stream increased and it began to rapidly downcut to a lower elevation. A stream terrace is simply an old floodplain that is now abandoned. This abandoned meander then forms a lake known as an oxbow.Īnother common feature of alluvial systems is the stream terrace. Once the neck is cut, the channel is much straighter, and the meander is abandoned to become a part of the floodplain. Finer sediments flow with the stream water out onto the flat area behind the levee, known as the floodplain.ĭuring the same flood, if the water is especially high, or the channel is highly meandering, the flood may cut a new channel, connecting two closely positioned meanders, a neck, in what is called a neck cut-off. That is why humans build man-made levees -to emulate natural levees. As the natural levee builds up over thousands of years, it helps prevent flooding. This forms a very gently sloping lump of alluvium that parallels the channel, known as a natural levee. Coarser sediments are therefore deposited very close to the channel. As the water flows out of its channel, it immediately begins to slow down because it spreads out over a large area, increasing the resistance to flow. When a stream floods, several processes naturally follow. On the inside of the meander, flow decreases, so deposition occurs a sand bar, or point bar, forms.
At the outside of a bend in a channel meander, the flow is concentrated and so erosion causes undercutting, and a cutbank forms. All channels naturally curve, or meander. The channel is the sloping trough-like depression down which water flows from the stream's origin, or head, to its destination, or mouth. Many stream systems consist of several common features including channels, heads, mouths, meanders, point bars and cut banks, floodplains, levees, oxbow lakes, and stream terraces. The conditions under which an alluvial system forms are found in both arid and humid climates, and in areas of both low slope (river deltas or swamps) and high slope (mountain streams).Īlthough the system mentioned above was in a mountainous setting, any river or stream is part of an alluvial system. Any collection of materials deposited by a process such as this is known as alluvium. The larger bedload materials (for example, rocks and stones) accumulate first, and the lighter suspended materials (sand, silt, and clay ) later. As the river slows down, suspended materials begin to be deposited. But then imagine that the stream rushes out of the mountainous region and onto a valley floor. As long as the stream is flowing rapidly, a considerable quantity of materials such as these can be transported, either along the bottom or as particles suspended in the water column. The basic principle underlying alluvial deposits is that the more rapidly water is moving, the larger the particles it can hold in suspension and the farther it can transport those particles.įor example, suppose that a river is flowing across a mountainous region, eroding rock, sand, gravel, silt, and other materials from the stream bed. An alluvial system is a landform produced when a stream or river, that is, some channelized flow (geologists call them all streams no matter what their scale) slows down and deposits sediment that was transported either as bedload or in suspension.
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