Why Don’t Rivers Run Dry?
What parts of the hydrologic cycle put water into the river?
Precipitation, infiltration, and runoff put water into the river.
What parts take water out of the river?
Evapotranspiration, which consists of evaporation of water from the river, and transpiration of water by plants growing in the water.
Can people increase or decrease any parts of the cycle? If so, how?
Yes. They can decrease infiltration and increase runoff by cutting down trees and clearing vegetation, as well as by paving over parts of the watershed. They also can plant different vegetation [such as food crops], which will change evapotranspiration compared with natural vegetation.
Is there anything in your neighborhood that changes the water cycle?
The answer to this question will depend on the individual neighborhood. Large amounts of pavement will make more of the precipitation runoff directly into the river. Vegetation slows runoff and more of the precipitation filters into the groundwater, where it may join the river later by way of springs.
Why Do Rivers Carry Stuff?
Why do you think river water might look muddy or murky?
Rivers transport materials, including mud and silt, that make them appear murky.
Does the muddy water in a river mean that it is polluted?
Not necessarily. River transportation of mud is a perfectly natural process. However, muddy water may also be polluted water. It is difficult to tell by simply looking.
What keeps particles in suspension longer, faster flowing water or slower flowing water?
Faster flowing water keeps particles in suspension longer.
How Do Rivers Make Their Beds?
Where does the stuff that rivers carry come from?
Much of it comes from the process of erosion, whereby material is weathered, dissolved, or otherwise worn away from the earth’s surface.
What kinds of landscapes produce the most eroded materials?
The 5-10% of earth's surface that is mountainous yields 80% of the sediment.
What happens when river water slows down?
The process of deposition. When water slows down, it can no longer carry as much stuff, and particles begin to settle out.
Which particles settle out first?
The large and heavy items settle out first. The very fine silts and organic materials settle out last.
What is the river bed?
The bottom of the river.
How do rivers make their beds?
They make their beds by eroding and depositing rocks, sand, silt, and organic materials.
What is the energy source for all of this work?
Flow, or water moving downhill, is the energy source. Note that some students may answer “the sun.” This is technically correct, because it is the sun’s energy that drives the water cycle, causing water to evaporate from earth’s surfaces. Once the water is up high, its natural course is to precipitate and then to flow downhill.
What kind of river bed would you expect in the upland portions of a river, and why?
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.
What is the floodplain?
The land on either side of a river, outside the banks of the river.
When the river gets out of its banks, what kinds of materials are likely to be deposited on the floodplain?
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?
How can humans change the normal balance between the natural processes of erosion and deposition?
By removing vegetation, disturbing the soil, and coating the land with impervious surfaces.
What happens to the river when humans take these actions?
Rainwater will run off much faster from the land and carry more soil with it into the river. The river may experience a surge in flow and erode the river bed more rapidly.
What is “hungry water” and how does it affect the river?
Hungry water is the swiftly-moving river water downstream of a dam that can carry a lot of materials. The river bed is eroded very rapidly in an unnaturally short stretch of river. The bed might be a lot rockier because most of the fine sediments are eroded away.
How do humans change the timing of river 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.