GEOSC 10
Geology of the National Parks

Rain, Rocks, and Rivers

Rain, Rocks, and Rivers

 

boats in the foreground, ready to launch into the Colorado River with cliffs on either side.
USGS scientists on the Colorado River in the Grand Canyon, AZ. The river naturally was very muddy most of the time, now is usually clear because mud is trapped behind the Glen Canyon Dam, but human-made floods sometimes make the river muddy, and are studied by USGS scientists.
Credit: USGS, Public Domain

The rivers of the Colorado Plateau are nearly as well known as their parks, for great rafting, incredible scenic views, and deep canyons. Rivers including the Colorado, the Green, the San Juan, the Fremont, the Virgin, and others have taken their place in history. But what are those rivers doing down there in the canyons?

Simply put, a stream or river is a conduit to take excess water, and sediment, from high places to low ones and usually to the ocean. Looking first at the water, rain or snow falls on the ground. (Streams are just smaller rivers, and streams are sometimes called runs or creeks or cricks or other names… and they all do more-or-less the same things, so you can use the terms interchangeably here.) Averaged around the world, rainfall (plus snowfall after it melts) is about 3 feet per year (1 m per year) (if you kept all the rain that fell in a year, and didn’t let any of the water evaporate or flow away or soak into the ground, it would make a layer about 3 feet deep). Pennsylvania's annual rainfall is also near the global average, as is the rainfall in much of the tree-covered eastern US. Some of this water evaporates directly, but most is used by plants and then transpires (evaporates) from their leaves. The evaporation from plants usually is lumped together with evaporation from other surfaces and called “evapotranspiration.” In a humid temperate climate such as central Pennsylvania, roughly two-thirds of the rainfall is returned directly to the sky by evapotranspiration; in dry climates, a larger fraction of the rainfall—often almost all of it—may be returned to the air by evapotranspiration.

Of the water that avoids evapotranspiration, a little actually falls on lakes or rivers, and some may fall on the land surface and then flow directly and rapidly over the surface into lakes or rivers, especially from the surfaces that humans have covered with buildings, roads, and parking lots, thus keeping the rain from soaking in. But most of the rain that avoids evapotranspiration soaks into the ground to form groundwater.

Soils and most rocks contain interconnected spaces, including gaps between grains of sand, cracks in the rock (which often are called joints), caves, and other openings. The ground acts a little like a sponge, with water soaking in and then slowly draining out to the rivers. We will discuss this groundwater flow a bit more when we visit Mammoth Cave, next; for now, simply note that because gravity pulls water down, rocks near the surface usually have some air in the spaces even where conditions are damp, and deeper rocks usually have all their spaces filled with water. The surface separating the deeper rocks with all spaces water-filled from those closer to the surface containing some air in spaces is called the water table, and where the water table intersects the surface of the Earth, a river or lake occurs.

Rivers flow even when it isn't raining because water is slowly draining through the ground from beneath hills to the rivers. The water table rises in elevation during wet times as the “sponge” of the Earth fills up with rain, and the water table falls during dry times as the sponge drains to keep the rivers flowing. And the water table is just below the surface in valleys and actually hits the surface at rivers, but you must drill deeper under ridges to penetrate the water table and complete a water well.

Video: Water Table (3:48 minutes)

The following video shows the movement of water in the atmosphere and land. Rainfall supplies evapotranspiration back to the air, runoff along the surface, and groundwater that flows through spaces in rocks to reach streams. The water table rises as rain fills the "sponge," and the water table falls between rains as the "sponge" drains to the stream.

Click here for a transcript of the water table video.

Let's have a look at water interacting with the ground. Here's a bird flying over a diagram that we're going to use to understand some of these features. This is what you can really think of. This is Shavers Creek Environmental Center of Penn State. This is a little stream that flows into Shavers Creek, just downstream of the dark, cliffy spot, which is well worth a visit if you get a chance.

But we'll go back to our diagram. You know that usually it is not raining, so we'll make the rain go away. And now it's not raining, but you need water for drinking and cooking and flushing your toilet. So maybe you need to drill a well. If you start drilling your well, initially you're going through rock or soil that may be damp, but there is some air in some of the spaces, and at some depth you will hit water.

You'll hit what we call the water table. Below that, you're drilling through rock or soil, in which all the spaces are full of water. You can pump that water out and use it. If you were to drill your well closer to the stream, you don't have to drill as deep. The water table is sort of a smooth version of the upper surface, and the water table hits the surface at streams and lakes.

Now we put the rain back in and ask what happens to it. A little runs off more if it's raining on a paved parking lot. But in natural systems, only a little bit of this goes into direct runoff directly to the stream. Most of it, two thirds in central Pennsylvania, starts to soak into the ground, but then it's grabbed by roots, or maybe a little bit just evaporates directly. Most of it grabbed by roots, goes through trees.

It then goes back to the air. We call that transpiration. Add in the part that evaporates directly and it's evapotranspiration. And sort of two thirds of the rain that falls in Pennsylvania just goes back up in the air where it can make new clouds and rain again. With two thirds evapotranspiration, only a little bit runoff that leaves almost one third to soak into the ground, eventually move through those water filled spaces, over to the stream and down the stream headed for the ocean.

Yep, there's a little coming in from the other side of our diagram too. When the water is reaching the water table faster than it flows away, you'll start to fill some of those spaces that had air just above the water table. And the water table rises in elevation. If it rises all the way across, it can make a flood, and we need to know about things like that. Most of the time, though, water is not arriving, but water is leaving down the stream.

And so, you start to drain water out of some in those spaces, and the water table drops in elevation. If it drops too far, the stream may dry out. If it drops too far, your well may dry out if you didn't drill it deep enough. And you may have to find a different source of water. So, back to Shavers Creek and a few of the things to know about water and the landscape.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Video: Grand Canyon Groundwater (1:43 minutes)

Water pumped out of the ground does not flow naturally to springs and rivers.  Sometimes, we use that water to flush our toilets or in other ways, and then dump the water back into the river farther downstream.  Other times, we use the water to grow crops or lawns and lose the water to evapotranspiration, so the water never reaches springs or rivers, which could dry up as a result. Here’s another vintage CAUSE video on how this sort of water use could impact nature at the Grand Canyon.  Such issues are increasingly important across the world.

Canyon Groundwater/Grand Canyon National Park
Click Here for a transcript of Canyon Groundwater Video

So now we're on the rim of the canyon. The rim of the canyon does not have streams on the surface. When it rains, which is not all that often, but when it rains, what happens is that the water soaks in. So if you had a rainstorm, you see the water start to make a stream, but then it soaks in. What happens to that water? It goes down through the spaces in the rocks until it hits a rock that it can't get through, such as the Bright Angel Shale. Then it runs along laterally and it comes out at those beautiful springs that we see in the canyon.

Now what's happening here is there's more and more people want to live here, and they want to have water, so they drill wells. And after you drill a well, you suck the water out and you use it to water your grass, or your crops, or your golf course, or to drink or flush or what have you, and eventually it evaporates. And so what you're doing is removing the water that should be running to the springs. And there's worry-- it's not happening yet, but there is worry-- that someday, we may start to lose the springs in the Grand Canyon and the unique biota, the wonderful ecosystems, that live down there because of development outside of the park up here.

Credit: © Penn State is licensed under CC BY-NC-SA 4.0

Rearranging Rivers

As we saw back at the Badlands National Park, weather attacks rocks to produce loose pieces through the processes that cause weathering. And as we saw at the Gros Ventre slide near the Grand Tetons, processes on hillslopes including soil creep and landslides deliver the loose pieces (which we can call sediment) to rivers. A river is then faced with a balancing act; it must transport both the water and the sediment delivered to it.

You may be able to think of many ways for the river to adjust if it receives more or less water, more or less sediment, bigger or smaller sediment pieces, or “stickier” or “less sticky” sediment pieces (those more or less likely to clump together). For example, if more sediment is delivered to a river than it can remove, then the sediment will pile up, raising the elevation of the bed of the river where the sediment is being delivered. This steepens the river flowing away from the pile—the elevation of the ocean where the river ends has not changed, but the elevation of the riverbed is higher above the ocean—so the river flows faster and is better able to move the sediment. If nature delivers more water and less sediment, the river will tend to wash away all of the sediment supplied and have energy left over to carve into the rock of the riverbed. This cutting downward will make the river less steep from there to the sea—because the river ends at sea level and can’t lower sea level, lowering the upstream reaches of the river must make the slope to the ocean less steep. A less-steep river will carry less sediment and so reduce or eliminate erosion of its bed, and often will flow over loose sediment without removing that sediment and reaching bedrock. Such a river tends to reach a balance in which it just removes the water and sediment supplied to it. In the process of reaching this balance, rivers also may adjust the width, depth, and shape of their channels as well as the steepness.

That background, it should not surprise you that rivers are highly diverse. A white-water rafter braving the rapids of the Grand Canyon sees a very different setting than a cruise passenger on the Mississippi River going to New Orleans! The white-water guide and the cruise ship captain have jobs that require understanding rivers, and so do hydroelectric-plant managers, fisheries experts, city planners, designers and architects trying to protect people from floods, water-supply professionals, and many others. When Europeans settled in North America, they often moved inland from the coast along rivers to establish towns that became cities, and trade moved along rivers. Thus, today many people live near rivers, and all of us interact with rivers, if only by traveling over them on bridges and trusting that the engineers understood the way that the river interacts with bridge foundations.

Each river is unique, but we can find a few repeating patterns that will help you see the bigger picture. We will briefly discuss three of them: straight, meandering, and braided.  

Straight Rivers  

The only way to make a truly straight river is to dig a trench and line it with concrete… and that really isn’t a river. But, in many cases, a river flows in a single channel, often eroded into bedrock. If you look upstream or downstream along such a river, the canyon walls often look a little like the letter V with the river flowing in the bottom. Such rivers are often eroding slowly into the bedrock, with the water able to carry away all the sediment that is supplied plus the little extra rock that they break loose from their beds.

The Colorado river at the bottom of the Grand Canyon
The Colorado River in the Grand Canyon is a great example of a straight river.
Credit: Colorado River by Alex Demas from USGS (Public Domain)

Video: Making a Sand Canyon / Grand Canyon National Park (1:30 minutes)

Downward erosion by a river into its bed produces steep river banks that then erode back, so you see the river banks making a sort of V if you look along the river, as in the picture of the Grand Canyon just above. In this vintage CAUSE video, Dr. Alley makes a very small-scale demonstration of how this happens, in sand near the Grand Canyon.

Making a Sand Canyon, Grand Canyon National Park
Click Here for Transcript of Making a Sand Canyon, Grand Canyon National Park Video

[MUSIC PLAYING]

If you make a canyon, and you deepen that canyon, the sides fall in. And one can see the sides avalanching in there merrily, eating back over time. See waves of adjustment going on. It's really beautiful to see.

I steepen it, and it steepens at the bottom. And then it eats its way back, and then it eats back at the head, eventually. Well, that's exactly what's going on here.

And so we can see across the river. The river is cut down. And then above it, there's that slope of rocks that have fallen off of the cliff. And as the river cuts down, that slope will be lowered and rocks will fall off of it and come avalanching down in exactly the same way as what we're seeing going on here in a very small scale. And you'd see all these little waves and nick points, and there are quite a number of very exciting things going on here.

Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Meandering Rivers

In addition to these “straight” rivers, we often see rivers in patterns called meandering and braided (see the pictures). These patterns are common in rivers that have received more sediment than they could move, so the river flows across some of its own sediment. A meandering river generally has a single, deep channel, but that channel flows in great curves or meanders. A braided river, in contrast, tends to be wide and shallow, with the water splitting and rejoining around many sand bars or gravel bars scattered across the river.

Much of the difference between the braided and meandering rivers is related to the type of sediment they carry. If you have ever worked with clay in a pottery class, you know that it is made of very small pieces (you cannot see or even feel the individual pieces, the way you can see and feel larger grains of sand or gravel or boulders), and those tiny pieces stick together well (you can make clay pots, or you could make and throw balls of clay). When the sediment delivered to a river is rich in these small clay pieces, their stickiness allows the river to form a single deep channel with steep banks that don’t collapse. When the river does knock some sediment loose, that sediment tends to stay suspended up in the water (we call this “suspended load”), which flows rapidly because it is far from the river bed where friction with tree roots and the bed itself slows the water.

Such deep streams typically curve back and forth, or meander, along their paths. Put a tree’s roots, or a boulder, or almost anything else in the way of the river, and the water flow will be deflected away from the obstacle, hitting the other bank of the river and eroding it, so once started, a meander bend will grow. Meandering rivers usually occur in relatively flat, lowland regions towards the coast, such as the Mississippi heading for New Orleans, but you can find meandering streams elsewhere. (Meanders even develop without obvious causes, as in some streams flowing on top of the ice of melting glaciers—something disturbs the flow, and then the pattern of hitting the other bank and eroding it or melting it on a glacier leads to meanders.)

very windy steam in a valley
Meandering stream draining into the fjord of Sondre Sermilik, South Greenland, viewed from a helicopter. The nearly flat bottom of the valley was carved by a glacier. After the ice melted, the stream has been meandering across the valley floor, as shown by the old, abandoned channels. Most meandering streams are in the lowlands, towards the coast.
Credit: R. B. Alley © Penn State is licensed under CC BY-NC-SA 4.0

Braided Rivers 

a river receives lots of sand and gravel or even bigger chunks rather than clay, the large pieces do not stick together well enough to form a steep river bank, and instead tend to collapse into the water. The single deep channel of a meandering river would become wider and shallower if its river banks collapsed, and a wide, shallow river is good at rolling sediment along its bed. Sediment rolled along a river bed is called “bed load”. Often, the bed load will be piled into a lot of sandbars or gravel bars in the river during a flood, and the bars will be left sticking out of the water as the flood ends. Water then must flow around these sandbars. When viewed from above, the splitting and joining of parts of the river around the sandbars looks something like ropes of water that have been braided together, so these are called braided rivers. They are common in upland regions, where steep mountain slopes shed landslides of coarse sediment into the channels, but you can find braided rivers elsewhere.

Whether straight, meandering, or braided, rivers move water and sediment downhill. And, when people build dams to form reservoirs for flood control, recreation, or other purposes, we interrupt the sediment transport as well as the water flow, with consequences that we discuss next.
 

large river in a Y shape
North Fork, Toutle River. The great amount of sediment in this braided river came from the eruption and landslide of Mt. St. Helens in 1980.
Credit: North Fork Toutle River by Adam Mosbrucker from USGS, November 2012 (Public Domain).