Rivers Move Sediment
“It has been said that streams are the gutters down which flow the ruins of the continents.”
Everyone knows that rivers move water. But what is not so widely appreciated is that water is one of only several things, important things, that move in rivers. We can’t talk about why rivers look the way they do, and how they change over time, without discussing two of these things: sediment and large wood (logs or whole trees). In this chapter, let’s focus on sediment. When most people hear that word, they think of muddy, turbid water. Or, perhaps the residue at the bottom of the wine bottle, or your coffee cup. But sediment, to a river scientist, includes a much wider range of solid material, from the indistinguishable grains of silt and clay that make water look turbid, through the gritty sands, gravels, and cobbles that commonly make up the river bed. The river moves all of these materials. And it is through moving these materials, in interaction with obstacles to flow, like logs, boulders, and the river bank, that the river attains the shape that we see, and changes over time, if we pay attention.
As water is moving down the river channel, it isn’t just gliding effortlessly. It’s actually sort of scraping its way along, creating a frictional force along the streambed. And the turbulence of the water is impacting the bed too, little pulses of water hitting the bed and knocking loose grains of sand, gravel, and even sometimes cobbles. Think of the water like a piece of sandpaper, carving up particles of wood and carrying them along as it moves. The frictional force dislodges sand and gravel from the streambed, and carries it, sliding, bumping, rolling, bouncing, in a downstream direction. Eventually, this sand and gravel is carried into an area where the river is much quieter, and the frictional force diminished. This can happen if the rain stops, and the water flow diminishes. Or, it can happen if the flow enters a part of the river that is flat, slow-moving. Then, grains of sand and gravel settle out again, becoming part of the streambed, immobile.
The name for this process, overall, is sediment transport. The material moving along the streambed by bouncing, rolling and bumping is called bedload. Finer-grained material, such as silt and clay, and in some situations sand as well, is buoyed up into the current by its turbulence, and basically moves downstream without touching the bottom. This material is called suspended load, and is responsible for the water looking turbid. As we will see later when we talk about how rivers form their channels, the bedload plays a key role in forming the river bed and determining the size and shape of its channel. The suspended load plays a key role in forming the floodplain soils.
Water that is moving faster, higher velocity water, can pick up and move larger grains of sediment. Higher velocity water also moves more sediment, greater volumes of sediment, over time. In fact, the amount of bedload moved over time turns out to be an exponentially increasing quantity as the water flow increases. Doubling the flow, for example, can increase the rate of bedload transport by four times in sand-bedded streams, and eight times or more in a gravel-bedded stream. Generally, in most rivers, bedload is not moving when the river is at low flow conditions, that is, when there is not a rain storm going on. In fact, bedload might only move, on average, on 10 to 15 days of the year. Since bedload only moves during a small portion of the time, and usually during storms and high water, measuring it, and recording it in video, can be tricky, and even dangerous. Here is an example of bedload moving in the Grand Canyon, where the flows have more to do with dam releases upstream than they do with storms:
https://www.youtube.com/watch?v=EGfRoyP1RHc
And here’s one of bedload moving in a coastal Alaska stream, which is moving bedload even though the weather is calm:
https://www.youtube.com/watch?v=jpexS4-9IF0&feature=youtu.be
There are a couple of interesting things to notice in these videos. One is that the bedload seems to move in surges, or pulses, driven by turbulent eddies that impinge on the stream bottom. Another is that all of the grain sizes that make up the streambed are moving, not just the smallest ones.
I mentioned earlier that the streambed is formed by bedload moving and depositing over long time spans. In fact, a sort of equilibrium develops between the exposed surface layer of the streambed and the bedload in motion. Grains of gravel and sand are constantly moving in from upstream, and coming to rest, while other grains are being stirred up and mobilized and carried off downstream. The streambed forms a coarsened surface layer in order to allow this interchange of grains to go on without net erosion or accumulation of sediment. This is called the dynamic pavement concept (the coarsened surface layer is sometimes called the pavement layer, but more usually, the armor layer). To explain how this works, I’m going to first tell you how it doesn’t work. That is, the older view of streambed dynamics that we now know isn’t accurate. I call this older, now defunct, story the “Shingled Roof in the Hurricane.”
We used to think that the coarsened surface of the streambed was like a shingled roof. During a hurricane, as the wind increases, the roof holds firm until a velocity is reached at which a couple of shingles become dislodged. Then a shingle blows away, and leaves an opening, and allows the wind to get in underneath the other shingles, and then...Whoosh! All at once, the roof blows off, and all the shingles are carried downwind. We thought that the streambed was like this. As the flow increased, a threshold would be reached where some of the larger grains making up the streambed surface would be dislodged. And then, suddenly, this coarsened surface layer (called the armor layer) would break up, and the entire streambed would be in motion.
But observations of streambeds under conditions where bedload is moving have shown that this is not the way it works. During most flows when bedload is moving, the armor layer never does break up. It stays intact! Even if the streambed temporarily erodes downward, and builds back up again (fills in) later, the armor layer rides up and down with it. What is happening is that as the flow increases, more and more grains in this layer are becoming active, like flashes of light sparkling across a dark backdrop, and which grains move is as much a function of the structure of the streambed layering itself as it is the force of the flowing water.
Normally, you would expect that progressively larger grains of sediment begin to move at progressively larger water velocities. In other words, a pebble 4 mm in diameter will be moving while pebbles 8 mm in diameter have not yet begun to move. There would be an orderly, linear, increase in the size of particles moving as the flow increases. Twice the flow would result in twice as large a particle moving. However, this armor layer creates something called a “hiding factor.” In the coarsened streambed, the larger grains stick out further into the flow, exposing themselves to the force of the high velocity water. Meanwhile, smaller grains are shielded from the flow by these larger grains, lying in their shadows and the crevices between. This causes an equalization of mobility. It causes the large particles and the small particles making up the streambed surface to begin moving at nearly the same flow. And so the streambed armor layer can become adjusted in its composition so that the grain sizes and quantities of sediment being mobilized equal the grain sizes and quantities being carried in from upstream. In this way, the stream can transport the coarse half, and the fine half, of its sediment load in balance, without eroding or filling in.
Continuing with the defunct analogy of the roof in the hurricane, it’s as if, instead of the entire roof blowing off when a few shingles begin to dislodge, there is a steady stream of shingles being carried by the wind so that when the shingle blows away, another shingle from upstream can land in the space left by the first shingle, so that the roof stays intact. And as the wind gets ever stronger, all that happens is that more and more of shingles on the roof are participating in this exchange, with greater and greater numbers of shingles being carried in the wind. This is the dynamic pavement story - a balance or equilibrium between material blowing in and blowing out.
Since the armor layer becomes adjusted to an equilibrium with the bedload, looking at that armor layer, and comparing it with what is underneath, can provide information about the long-term, average, sediment transport of the stream, which is usually referred to as the sediment load. It turns out that the material underneath this coarsened armor layer is representative of the long-term average composition of the bedload. In other words, if you were to capture the whole year’s worth of bedload in a big box, and shake it all up until it was well mixed, it would look like this subsurface material in terms of the relative mixture of grain sizes present. If the surface armor layer looks really different from this, that is, has a much greater prevalence of larger grain sizes, that generally indicates a low sediment load. On the other hand, if the armor layer and the subsurface layer are rather similar, that tells us that the sediment load is high. In that case, the streambed doesn’t have to adjust very much to stay in equilibrium, because there is so much bedload moving.
Another thing to notice about the streambed layering is whether the large grains are in contact with each other or not. Streambeds in which the large grains touch each other, with the smaller grains filling in the gaps between large grains, are called framework-supported, and usually occur where sediment loads are low to moderate. But if the large grains do not touch each other, and instead are basically surrounded by a soup of finer grained material, this is called matrix-supported. Streambeds that are matrix-supported are indications of very high sediment load, especially when the gravel sediment load is mixed with large amounts of sand. We will talk about bedload movement in purely sand bedded streams in a later chapter.
If the sediment load becomes very small, such as downstream from something that blocks all of the sediment, like a dam, then the streambed armor becomes very coarse. Eventually, it becomes so coarse that it doesn’t move much at all. The streambed becomes frozen in place, fossilized. As you might imagine, this has huge impacts on creatures that depend on being able to dig into or burrow into the streambed, like spawning salmon.
References:
Luna B. Leopold, M. Gordon Wolman and John P. Miller, 1964. Fluvial Processes in Geomorphology. W. H. Freeman and Co., San Francisco, CA. 522 pages.