What is an Alluvial River?
Mil Creek, near Walla Walla, WA
“The river, then, is the carpenter of its own edifice.”
- Luna B. Leopold, 1994, A View of the River (page 281)
In a separate article (Types of River Channels), I mentioned that rivers come in two basic types: alluvial and non-alluvial. Recall that alluvial rivers have streambeds and streambanks made out of gravels and sands of the river itself has moved and deposited over time. The word “alluvial” means carried by moving water. What makes alluvial rivers interesting is that they attain a form or shape which reflects a balance of the process of erosion and deposition. They don’t just cut their way into the landscape, the way a non-alluvial river does; they actually build their own landscape. Let’s discuss these features of alluvial rivers in more detail.
One characteristic of alluvial rivers is that they are almost never straight. They curve or meander from side to side as they flow down the valley. Scientists use the term sinuosity to describe this. The greater the sinuosity, the greater the actual distance the water has to flow, down its curving, circuitous path, to move another mile or kilometer down the valley.
Another characteristic of alluvial rivers is that they have active floodplains. At least, this is true of those rivers that have had the time and opportunity to develop an equilibrium of sorts between erosion and deposition of sediment. An active floodplain is a relatively flat land surface adjacent to the river channel which is inundated on a regular basis, that is, a couple of times per year at least*. And when the river inundates the floodplain, it carries in suspension silt and sand, which deposits on the floodplain as the water spreads, gets shallow and slows down. This, in fact, is how floodplains form. Floodplain soils are alluvial soils, and the floodplain itself is a mixture of abandoned river channels which have become filled in, and these flood deposits. The floodplain is remarkable in that it gives the river room to move around. Over decades and centuries, the river channel can migrate from one side of the floodplain to the other.
Why do rivers meander? Scientists have struggled with how to simply explain meandering, even though it is a consequence of basic physics. In fact, the great scientist Albert Einstein once wrote an essay trying to explain how rivers meander. He got it wrong, though, attributing meandering to the Coriolis force, the phenomena which causes Northern ocean currents to veer to the right, due to the Earth’s rotation. But this Coriolis force is very small relative to the hydraulic forces at work in turbulent river water.
Try to imagine a straight, alluvial river. This river is not confined between difficult-to-erode hillslopes, but is flowing down a broad floodplain made of material that the river can easily transport. The water flow in this river channel is turbulent, as it is in all rivers. Small swirling eddies of water are everywhere, and irregularities in the channel bottom or sides deflect the water to amplify this turbulence and create new eddies. In some places, this turbulence focuses the water’s erosive energy on one side or the other, causing the river to erode on that side, and accelerating the amount of water flowing along this side as the channel gets deeper next to this eroding bank. The bank is no longer straight but develops a curvature as it erodes outward, and this curvature must turn the water flowing against it, back towards the channel center. As the water turns, the centrifugal force, the same force that causes your bag of groceries to tip over in the car as you go around the corner, flings the water near the surface into the direction of the bank, piling the water up. This piling up of the surface water causes water along the bottom of the channel flow back towards the channel center, to relieve the pressure. The net result of this is that the water flows around the bend in a spiral fashion, descending along the bank and moving crosswise along the channel bed towards the center.
Meanwhile, on the shallower opposite side of the channel, the water velocity will be much lower, allowing sediment to deposit, building a gravel bar. The gravel bar formation is amplified by the spiral flow of the main current, as it tends to move the eroded material away from the outer bend and back towards the opposite side, dropping some of that material on the end of the bar. In this manner, natural turbulence, combined with the river’s ability to erode transport and deposit sediment, have created a meander bend and opposing gravel bar, called a point bar.
This type of meandering alluvial river is called a pool-riffle channel. The riffle refers to the shallow area at the downstream end of the point bar, where the water is moving out of the meander bend, and heading into the next meander bend to curve in the opposite direction. The sediment making up the river edges and bottom can be either sand, or gravel, or both. There are many differences in the way the river moves sediment in sand- versus gravel-bed channels, but the end result is the same: a pool-riffle channel.
In an alluvial river, the size or dimensions of the channel are not random. The river erodes and deposits in a way that creates a channel with a certain size and certain proportions that is determined by the amount of flowing water during times when sediment is moving (in particular, the small but frequent floods that move most of the sediment) and the amount of sediment that must be moved (the sediment load, the amount that gets into the flow from the watershed upstream).
The size of the channel, that is, its cross-section, is just big enough so that the channel can move most of its sediment load when the water level is right at the top, just about to be spilling onto the floodplain. Scientists use the term bankfull flow to describe this condition. The proportions of the channel can be described by reference to this bankfull elevation. The average bankfull width, for example, determines the meander geometry. Just as a human body tends to have consistent proportions, such as arm length, relative to the overall body size, a river’s dimensions scale with bankfull. For example, the distance from one meander bend to the next one on the same side downstream will be between 5 and 7 times the bankfull width of the channel. By studying many stream channels throughout an area of consistent climate and geological conditions, scientists have developed relationships allowing them to predict the average dimensions of alluvial river channel (widths, depths, cross sectional area, sinuosity, average bankfull depth and bankfull water velocity) from the watershed area upstream. Scientists call these relationships the hydraulic geometry of the river, or sometimes the bankfull geometry.
Why does this work? It’s all about equilibrium, balance between opposing processes (erosion and deposition), and the way that the geometry of the river channel affects these processes. The deeper the channel, or the higher elevation of the floodplain, the more water that the channel can contain during moderate floods. More water means greater hydraulic forces, which means greater erosion and greater amounts of sediment transported. A shallower channel spills water onto its floodplain at a lower flood discharge, which reduces the hydraulic forces in the channel available to transport sediment. Likewise, the more sinuous the channel, the greater the arc of each meander, the longer the distance the water must flow in order to drop down the valley slope. Longer distance means less slope within the channel, which reduces the gravitational force moving the water, and in turn reduces the water’s capacity to move sediment.
Although a blow-by-blow description of how the channel adjusts itself would be complex, what basically happens is that there is natural feedback built into the system. If erosion is out of balance with deposition, the channel, eventually, self-adjusts its dimensions to bring these processes back into balance. This is the physical explanation for why the geometry and proportions of river channels, alluvial channels that is, are predictable.
Scientists have also discovered that the geometry of the river is adjusted so that the total hydraulic energy dissipated, through the work required to move sediment and the work required to overcome friction as the water scrapes against the banks and streambed, is minimized. Not only is the overall energy expenditure minimized, but the distribution of energy expenditure throughout the length of channel becomes uniformly spread out or equalized.
A consequence of these physical tendencies to attain both minimum energy dissipation and equalized energy expenditure is that the classic geometry of a meandering river can also occur in many unexpected places throughout the world. Ocean currents, such as the Gulf Stream, attain this same meandering shape, and obey the same proportional relationships. Water flowing on the surface of glacial ice will, if given long enough time, develop a meandering form along the same proportions, even though the actual process involved (melting of ice) is different from erosion of sediment.
As a final caveat, we must always realize that rivers are more than just moving water and sediment. They also interact with vegetation, and in forested environments, trees that produce logs and logjams will have a huge impact on the shape of the river, including creating other river geometries that have not been mentioned in this article. Readers who are curious to learn more of the technical details of how alluvial rivers “are carpenters of their own edifice” are invited to read the delightful book, A View of the River, by Luna Leopold, who was quoted at the beginning of this article.
Notes:
*Notice that this definition is not the same as the one used by insurance companies and tax assessors, the “100-year floodplain.” The 100-year floodplain is purely a human, mathematical invention, and has no relevance to the ecology or formation of rivers.
References:
Luna B. Leopold, 1994. A View of the River. Harvard University Press, Cambridbe, MA. 298 pages.