Why Don’t Rivers Run Dry?
If you have ever had occasion to sit on the bank of a river and watch it for a while, you may have gradually become aware of the astonishing volume of water moving past you. Day and night, every day of the year, rivers flow downhill to the sea. No other ecosystem has such an unconcealed physical manifestation. And no other ecosystem is so inscrutable to scientists and irresistible to poets and song writers. The lonely 19th century American poet Emily Dickinson penned this verse:
My River runs to thee—
Blue Sea! Wilt welcome me?
My River waits reply—
Oh Sea—look graciously—
I’ll fetch thee Brooks
From spotted nooks—
With a very few exceptions, rivers don’t run dry. This is because they are part of the vast global hydrologic cycle. The term “hydrologic” is from the Greek hudor—meaning “water.” In this cycle, water moves from sky to earth and sea and back to sky again. Powered by the sun’s energy in the form of heat, water continuously circulates in the earth-atmosphere system.
Why Do Rivers Carry Stuff?
One of the more noticeable things about rivers is that the water is almost never perfectly clear. Rivers may appear cloudy, muddy, or murky. This does not necessarily mean that the water is polluted. Instead, it may reflect a perfectly natural process: transportation of materials.
When silt, organic particles, and other things wash into the river, the flow of the water will keep them in suspension for a while. How long they stay up in the water depends on how fast the water is flowing and how large and heavy the materials are. As common sense suggests, faster water keeps items in suspension longer. Smaller and lighter particles stay in suspension longer than heavier particles. By the way, air behaves the same way as water in terms of carrying things. This is why air moving very fast in tornados or hurricanes can carry really large items over pretty fair distances and even gusty breezes can pick up dust and light sand. Rivers can carry a LOT of materials. The weight of materials that a river can carry may shock you.
How many days do you think it would take your class to carry one day’s worth of the sediment carried by the Mississippi River? At St. Louis, this would be about 273,973 metric tons, or 604,000,000 pounds. Assume that you and your classmates will load your backpacks with 10 pounds of materials, eat a good breakfast, then walk as far as the river moves in a single day, dump out the backpacks, and start again back upstream. If there are 25 students, you all could move 250 pounds per day. Thus, it would take you over 2.4 million days, or 6.6 thousand years to do the work the Mississippi does in a single day.
How Do Rivers Make Their Beds?
What happens to all of the stuff that rivers carry? And where does it come from in the first place? In order to answer these questions, we need to talk about erosion and deposition. The term “erosion” comes from the Latin erosus, meaning “eaten away.” Erosion in nature is simply the process whereby material is weathered, dissolved, or otherwise worn away from the earth’s surface. Although wind, glaciers, and waves breaking against the coast may be important in certain places, running water erosion is the most significant globally. This is how a lot of the materials carried by rivers get there in the first place. The factors that determine how much is eroded are climate, vegetation, lithology (what kinds of rocks), and relief (how hilly it is). The 5-10% of earth's surface that is mountainous yields 80% of the sediment!
The opposite process is deposition. The term “deposition” is from the Latin depositio, meaning lay down. When water slows down, it can no longer carry as much stuff, and particles begin to settle out. The large and heavy items settle out first. The very fine silts and organic materials settle out last.
Rivers literally “make their beds” (the river bed is the bottom) by eroding and depositing rocks, sand, silt, and organic materials. The energy source for all of this work is flow: water moving downhill. Where flow is very fast, such as in the upland portions of a river, the river bed will be very rocky because fast-moving water carries the fine sand and silt downstream but is not strong enough to move the underlying rock that makes up the mountain. Where the flow slows in pools, man-made reservoirs, or lowland portions of the river, the river bed will be made of much finer materials such as sand and silt.
When the river gets out of its banks, it floods land on either side. This land is called the floodplain. Like the river bed, the floodplain will reflect the amount and composition of sediments carried by the river. Sediments that are deposited on the floodplain are generally relatively fine, because the water moving across the floodplain will be traveling more slowly than that in the main channel.
How Do Humans Influence River Beds?
Humans can have a big impact on river beds by changing the natural processes of erosion and deposition, and the balance that ordinarily exists between them. For example, where humans remove the vegetation or disturb the soil, rainwater will run off much faster from the land, and carry more soil with it into the river. The Soil Erosion website has some dramatic pictures that illustrate these processes. Likewise, where humans coat the land with lots of impervious surfaces such as parking lots, highways, and buildings, rainfall will be delivered swiftly to the nearest river because the land cannot soak it up. When a lot of water is delivered in a short time, the river may experience a surge in flow, and erode the river bed more rapidly.
When humans build dams on rivers, the river water backs up behind the dam into a reservoir, which is a slow-moving body of water. Remember that slow-moving water cannot carry as much stuff, so sand and silt drop out into the bottom of the reservoir. Downstream, scientists say that you find hungry water, by which they mean the swiftly-moving river can now carry a lot more materials.
Hungry water erodes the river bed until it is carrying as much as it can. This erosion is a natural process, but it is occurring at an unnatural rate and in an unnaturally-short stretch of river.
Humans also change the timing of water flow patterns by using river water for irrigation and returning it to the river at some time other than when it might have been expected there. For example, if a dam collects spring runoff (that would ordinarily be flushing sediment downstream in the spring) and the water is used for irrigation in the summer and returned to the river, there can be significant changes in the frequency and timing of both high-water and low-water periods. This may directly affect erosion and deposition. It certainly influences the ecology of the river.