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How a River Flows

     Rivers always flow downhill, of course! A stream, or a river, is formed whenever water moves downhill from one place to another. This means that most rivers begin high up in the mountains, where snow from the winter, or ancient glaciers, is melting. On their way down to the sea, they collect water from rain, and from other streams. There is another place where rivers rise up: from springs. Where groundwater seeps up onto the surface, it may form a lake or pool or it may start running downhill right away, but one way or another, it is a river's beginning.

     In lower places where there are no lofty mountain peaks covered with snow, rivers often begin this way, springing up--well, from springs!

     What you see when you look at a river, though, is not the whole story. For the river is flowing even where you can't see it. It flows deep beneath the bottom of the river (the substrate), and it flows underneath the ground on both sides of it. If you are standing on the bank of a river, the river may well be flowing under your feet. This area underground where the river flows is called the hyporheic zone (hy-po-ree-ik). In the hyporheic zone, many forms of life can be found. In fact, a lot of the mayflies and other insects and crustaceans that we've been talking about can move down into the hyporheic zone. Hiding down there between the rocks protects them from predators, yet the water still flows to them, bringing them nutrients.

     Depending on how gravelly or rocky the ground is, a river's hyporheic zone has been found to extend far away from the part of the river you can see, far into the river's floodplain. In fact, wells drilled into the earth for houses near rivers will sometimes yield up tiny little animals! The more gravelly the ground, the farther the hyporheic zone will extend. In reality, a stream, its underground hyporheic zone, and its aquifer beneath the hyporheic zone are all part of a single system.

     Aquifers are sometimes called "water tables," though a water table is actually just the top margin of an aquifer. Under your feet, there is always a vast field of water. This is an aquifer, sometimes called groundwater. Some aquifers, like the Oglalla aquifer, cover large regions of the country. Water that has fallen as precipitation and soaks into the ground eventually reaches a level that it has a harder time soaking into. It might be shale or limestone, for instance. Since it can't flow straight down anymore, it moves sideways--but still downhill. This means that huge sheets of fresh water are slowly moving underneath your feet. They are, in effect, underground streams.

     It is important to realize, though, that an aquifer is not your ordinary stream. Because it is underground, its water fills the tiny spaces between particles of rock, dirt, and clay. The earth becomes a giant sponge, in other words. So while the water in an aquifer flows downhill, it does so very...very...slowly.

     There is often more than one aquifer in any given location. For instance, if you dig down, you may find an aquifer six or eight feet down. Your well might be sunk to an aquifer that is much deeper than that.

     Where aquifers reach open air, we no longer call them aquifers. If they run in channels over the land, we call them streams and rivers. If they are contained and slow, we call them lakes and ponds. Where aquifers "leak" out onto land, they make springs.

     Aquifers aren't always under your feet. They are in hills, too, so if you stand next to the hill you can look up at the aquifer. You can often tell where an aquifer leaks out on a hillside because the vegetation changes. For instance, along a hillside of sage and juniper, you may see a long line of bright green cottonwoods growing. This line is called the springline.

     So streams receive water not just from precipitation and snowmelt, but also from aquifers--and if an aquifer goes dry (or is drawn down by wells), its stream can dry up. These streams will follow their aquifer as it gets drawn down, cutting deeper and deeper into the ground until they hit bedrock or hard clay. (This happens because as the sides and bottoms of a stream dry up, they lose their plant communities and become vulnerable to erosion. New flows after drying spells erode both streambanks and streambeds). Many streams of the Southwest U. S. have dried up due to well-drilling to support larger populations. One of these, the Santa Fe River, has become a wide, dry arroyo with vertical walls up to twenty feet high in places. This has happened because very few people recognize that aquifers and rivers are part of the same hydrologic cycle.

     Below is a simplified diagram of part of the hydrologic cycle. Because most people are familiar with how water runs over the ground and into streams, that part of the cycle is not shown in this diagram. Instead, special attention has been given the role of aquifers.

hydrologic cycle
A Typical Watershed

     Looking at the blue vertical arrows, you can see that as snow melts and rain falls, water enters the aquifer. This is recharge. Because of adequate precipitation and cool temperatures in the mountains, you can see that the mountains have a snow pack--a large accumulation of snow that will melt slowly in spring, leading to even more recharge. The water in the aquifer then flows beneath the land the same way water would flow on top of the ground (but a lot slower). When it reaches a stream (or marsh or lake) it discharges into it. Water evaporates from the river and transpirates from the trees and other greenery, bringing more clouds and thus more rain.

     Below is a diagram of the same watershed. This time it is heavily developed, with a city, suburbs, and homes in the mountains, along with parking lots and roads. Very little water can soak into the ground and recharge the aquifer anymore because asphalt, concrete, and roofs don't absorb water. Wells (vertical red lines) are drawing the aquifer down. Large city wells by the river are creating cones of depression by the river. As a result, water no longer discharges from the aquifer to the river. Instead, the river is being drawn into the aquifer. During a drought, the river will dry up. And during a heavy rain--because the rainwater flows over the surface instead of soaking into the aquifer--the river will catastrophically flood. The city below is experiencing a drought. There is only a little snow in the mountains that will likely melt too quickly in the heat and run off over the land. It is easy to see that if the city continues to pump groundwater, it will find that its wells have run dry. Unfortunately, cities always respond to droughts by sinking more and deeper wells. The first casualties of the drought, then, will be the animals that live in the river. This city has set itself up for water restrictions and flooding.

hydrologic cycle
A Developed Watershed in a Drought

     Standing on a river's bank in summer, you can see where it flows high during winter: that will usually be where the rocky part ends and the mud or dirt and grass begins. However, all rivers will go even higher than that when the snow melts too fast or rain has fallen very heavy: they flood, and the area that they flood is called their floodplain.

river & floodplain

     It is easy to see a river's floodplain.  Like in the picture above, the trees and other plants
near the river, on its floodplain, are different from the ones standing further away from it.  
For instance, the land around you may be covered with dark green pines;  the river's floodplain will
provide quite a contrast with its bright green cottonwoods and willows.  There is a simple explanation for this sharp difference in vegetation:  some trees 
just can't abide having their feet get wet.  Often you will see young trees such as 
pines and Douglas firs growing on floodplains, but at the first flood they will
die, if they are not just washed away.
In the Pacific Northwest, 
a floodplain may hold black cottonwoods, alders, and of course, willows.  In old-growth forests, 
cedars and Sitka spruce will stand near rivers in the damp soil.  Also, because of year-round moisture, 
you will see lots of green grass and flowers near rivers.  If you walk into a floodplain, you will often find swamps or marshes as well--sometimes by accidentally stepping into them.  These are a part of the river, too, even when they are filled with plants, algae, and mud, and even if you can't see the water moving.  Marshes and swamps are valuable parts of a river that provide places for baby fish to hide and for other animals, from mosquitoes and flies to amphibians and crayfish, to grow and become food sources for fish and birds.

     The areas of special vegetation that grow along the sides of rivers are called the river's riparian zone. Riparian zones are critical to the health of rivers.

     One way rivers obtain nutrients is by changing their courses, and this they do all the time, although so slowly that we can't see it happening while we watch. Rivers meander, and the force of the water that flows through them is powerful, pulling and eroding away banks as the river moves unpredictably one way and then another. This meandering, of course, is pretty much restricted to their floodplains.

     Rivers have to meander: that is how they renew themselves. By meandering, they wash plants and soil from the land into their waters, and these serve as nutrients for the plants in the rivers.

     In their natural state, rivers flow fairly continuously. When flows decrease, for example during a drought, the decrease is slow enough to allow animals and plants that rely on the river to adapt by moving elsewhere or to utilize genetic adaptations built over millions of years. Rivers rarely dry up because aquifers continue to discharge into them. Flow changes are gradual because rainwater soaks into the ground and fills up swamps and marshes. Rainfall and snowmelt will cause temporary and gradual but not catastrophic flooding. Many of the stream's inhabitants depend on this predictable, periodic, and minimal flood cycle to stimulate reproduction. The flood cycles also enrich the river's riparian zone.

     Unfortunately, most rivers no longer have these gradual changes in flow. People have altered the way rivers flow in five major ways:

     Where there are dams, urban runoff, and artificially stabilized banks, it is easy to see that riparian zones disappear. Since riparian zones are vital to good stream health, this is destructive to stream life. And since stream life depends on continuous flows with gradual changes, including seasonal floods, it is destructive to stream life in another way. These human modifications are also dangerous for people, since they lead to streams cutting deeper and wider channels and periodic, devastating floods. Both destroy property, and the latter takes human lives as well. Besides the immediate danger of modifying rivers, flood-control methods harm people in other ways, by starving the land of nutrients and preventing the recharge of aquifers.

To learn more about how channelizing and damming effects watersheds, plants, animals, and people, visit the Ecology page and the Dams page.

     Above is a diagram of a river. As you can see, it looks very much like a tree. The smallest "twigs" of the river tree--here in bright green--are the small streams where the river begins. These tiny streams are called first-order streams. Wherever two first-order (green) streams join--here shown in white--they make a second-order stream. Where two second-order (white) streams join, they make a third-order stream (here in blue), and so on and so on until the river finally flows out into the sea.

     This is only a simple system of categorizing streams so they can be discussed. What it is missing are the countless tiny rivulets that join to form a first-order stream.

  • How many third-order streams do you see in this diagram?
  • Can you find the fourth- and fifth-order streams?

Play a Watershed Game! Be an Explorer or a Detective!

ball graphic Video: The amazing story of how wolves changed riparian zones in Yellowstone NP, which led to changes in river flow: "How Wolves Change Rivers."

ball graphicAn in-depth explanation of the hydrological cycle and stream flow from the U. S. Geological Service.

Reading listTo learn more about hydrology, check out the reading list, where you will find some excellent references listed. (You will be able to order them from here, too).

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When Streams Dive Underground

Most streams don't just meander from side to side. They meander up and down, too. This becomes obvious in desert areas, where the riverbeds are sandy and drought is a problem. As the level of a desert stream drops, places where it was at the bottom of its "wave" dry up, and all you can see is sand in the riverbed. But if you walk downstream a ways, you will come to where it was at the top of its "wave," and you can still see water in that section. Water is flowing in both places, of course--it's just that upstream where you saw it was dry, it is now flowing only underground.

In reality, streams actually "dive" down into the ground, into the hyporheic zone, and then come back up again further downstream, only to dive underground again even further along.

The Riparian Corridor

Often the greatest contributor of plant food to streams is the riparian zone: the margins along the stream that are filled with vegetation. These plants, like all plants, drop their leaves--which fall into or are washed into the stream. This is allochthonous matter (from outside the stream), as opposed to autochthonous matter (from inside the stream, like algae and diatoms). These leaves can't make oxygen, since they are dead, but they provide food to the creatures in the stream. Not only the leaves themselves can be eaten, but also whatever bacteria or fungus is covering the leaves, rotting them. It is this bacteria and fungus that is what crayfish are really after when they eat decaying plant matter. Riparian plants also have bugs on them which drop into the stream and provide food to stream-dwellers.

Riparian areas are valuable for more than just food. Riparian plants--like willow, alder, cottonwood, birch, cedar, alligator juniper, locust, and many others--help stabilize banks with their roots, provide shade from the sun (which gives the stream more complex habitats and helps keep it cool), assist in providing the correct stream acidity, absorb toxins, provide large woody debris (LWD) in which stream-dwellers can live, absorb floodwaters, assist in recharging aquifers, and help retain sediment (which keeps streams clean).