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. 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. For example, suppose that a river is flowing across a mountainous region, eroding rock, sand, gravel, silt, and other materials from the stream bed. 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. But then imagine that the stream rushes out of the mountainous region and onto a valley floor.
As the river slows down, suspended materials begin to be deposited. The larger bedload materials (for example, rocks and stones) accumulate first, and the lighter suspended materials (sand, silt, and clay) later. Any collection of materials deposited by a process such as this is known as alluvium. 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).Although the system mentioned above was in a mountainous setting, any river or stream is part of an alluvial system.
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 channel is the sloping trough-like depression down which water flows from the stream’s origin, or head, to its destination, or mouth. All channels naturally curve, or meander.At the outside of a bend in a channel meander, the flow is concentrated and so erosion causes undercutting, and a cutbank forms. On the inside of the meander, flow decreases, so deposition occurs; a sand bar, or point bar, forms.
When a stream floods, several processes naturally follow. 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. Coarser sediments are therefore deposited very close to the channel. This forms a very gently sloping lump of alluvium that parallels the channel, known as a natural levee. As the natural levee builds up over thousands of years, it helps prevent flooding. That is why humans build man-made levees—to emulate natural levees.
Finer sediments flow with the stream water out onto the flat area behind the levee, known as the floodplain. During 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. Once the neck is cut, the channel is much straighter, and the meander is abandoned to become a part of the floodplain. This abandoned meander then forms a lake known as an oxbow.
Another common feature of alluvial systems is the stream terrace. A stream terrace is simply an old floodplain that is now abandoned. Abandonment occurred when the erosive power of the stream increased and it began to rapidly downcut to a lower elevation. The stream did not have time to erode its old floodplain by meandering over it, so it was preserved. The abandoned floodplain, or stream terrace, can be seen well above the new stream channel elevation. Multiple terraces can sometimes be seen, resembling steps in a giant staircase.
When an alluvial system operates over a long period of time, perhaps millions of years, it works to flatten the surrounding landscape, and significantly decrease its average elevation. Areas that were originally mountainous can be worn down to rolling hills, and eventually produce extensive plains composed of alluvial sediment. The sediment is eroded from highlands that may be tens, hundreds, or perhaps thousands of miles from the coast, and the alluvium serves to bury existingcoastal features beneath a blanket of sediment. During periods of lower sea level in the geologic past, coastal plains extended far out on the margins of the continents. Today, these alluvial sediments are hundreds of feet below sea level.
As a stream emerges from a mountain valley, its waters are dispersed over a relatively wide region of valley floor. Such is the case, for example, along the base of the Panamint Mountains that flank California’s Death Valley. Astream flowing down a mountain side tends to deposit heavier materials near the foot of the mountain, somewhat lighter materials at a greater distance from the mountain, and the lightest materials at a still greater distance from the mountain. Often, the flow of water ends within the deposited material itself. This material tends to be very porous, so water is more likely to soak into the ground than to flow across its surface.
Thus, there is no preferred direction of deposition from side to side at the mouth of the stream, and as the alluvium accumulates it forms a cone-shaped pattern on the valley floor known as an alluvial fan. The idealized model described above would suggest that an alluvial fan should have a gradually changing composition, with heavier materials such as rocks and small stones at the base of the mountain and lighter materials such as sand and silt at the base (toe) of the fan. In actual fact, alluvial fans seldom have this idealized structure.
One reason for the more varied structure found in a fan is that stream flows change over time. During flows of low volume, lighter materials are deposited close to the mountain base on top of heavier materials deposited during earlier flows of high volume. During flows of high volume, heavier materials are once more deposited near the base of the mountain, now on top of lightermaterials. Avertical cross-section of an alluvial fan is likely to be more heterogeneous, therefore, than would be suggested by an idealized depositional model.
Alluvial fans tend to have small slopes that may be no more than a foot every half a mile (a few tenths of a meter per kilometer). The exact slope of the fan depends on a number of factors. For example, streams that drain an extensive area, that have a large volume of water, or that carry suspended particles of smaller size are more likely to form fans with modest slopes than are streams with the opposite characteristics.
Under some circumstances, a river or stream may continue to flow across the top of an alluvial fan as well as soak into it. For example, the volume of water carried during floods may cause water to cut across an alluvial fan and empty onto the valley floor itself. Also, over time, sediments may become compacted within the fan, and it may become less and less porous. Then, the stream or river that feeds the fan may begin to cut a channel through the fan itself and to lay down a new fan at the base of the older fan. As the fans in a valley become more extensive, their lateral edges may begin to overlap each other.
This feature is known as a bajada or piedmont alluvial plain. In some regions, piedmont alluvial plains have become quite extensive. The city of Los Angeles, for example, is largely constructed on such a plain. Other extensive alluvial systems can be found in the Central Valley of California andalong the base of the Andes Mountains in Paraguay, western Argentina, and eastern Bolivia.
Alluvial fans have certain characteristics that make them attractive for farming. In the first place, they generally have a somewhat reliable source of water (except in a desert): the stream or river by which they were formed. Also, they tend to be relatively smooth and level, making it easy for planting, cultivating, and harvesting. Deltas are common alluvial features, and can be found at the mouths of most streams that flow into a lake or ocean.
When rivers and streams flow into standing water, their velocity decreases rapidly. They then deposit their sediment load, forming a fan-shaped, sloping deposit very similar to an alluvial fan, but located in the water rather than on dry land. This is known as a delta. Deltas show a predictable pattern of decreasing sediment size as you proceed farther and farther from shore.
The Mississippi River Delta is the United States’ best known delta. Other well-known deltas are the Nile Delta of northern Africa and the Amazon Delta of South America. When Aristotle observed the Nile Delta, he recognized it was shaped like the Greek letter, delta, hence the name. Most deltas clog their channels with sediment and so must eventually abandon them. If the river then flows to the sea along a significantly different path, the delta will be abandoned and a new delta lobe will form. This process, known as delta switching, helps build the coastline out.
