This probably sounds like a silly question - but I’ve been thinking about this and I’m getting myself all confused and twisted up in a loop.
Rivers flow downhill. This is because the riverbed itself is generally sloping downhill. Right?
But the river surface, is that sloped too? Or is it on sort of step levels?
I try to think about this, and I imagine water being in columns, or in slabs, or in droplets, or… something else.
I guess it averages out and the end result is mostly sloping, but the mostly consistent level of the top of the river surface doesn’t seem to match up with the uneven level of the riverbed. And that’s the part that confuses me.
Is there a simple description that will put my mind at rest?
I expect Leonardo Da Vinci studied this kind of thing, it seems like the sort of stuff that would fascinate him…
However, you usually can’t see this since the width of most rivers constantly changes. That slows down and speed up the current, which causes the depth to change.
It might help to look at a good topographical map. It will show you that the river falls at a definate rate as measured on the surface of the water. Some rivers fall at the rate of mere inches per mile, while others fall at many feet per mile.
You could measure the fall of the river from the bottom of the river bed as well, by running a series of levels along it’s length, but it would require a lot more work and make you hold your breath a long time.
Seriously though, there is often a lot of activity on the bottom of a river, with the terrain constantly changing by the currents and sediment deposits. Things tend to balance out though, because the surface of the water pretty much remains constant.
Even lakes are sloped, since they have water coming in, and, usually, out. The amount of slope of a river is so small it is not discernible, except like in the case of a waterfall, or of rapids.
If this were the case then the water at the downstream edge of one of these steps would exert more force than the water upstream and the current would flow faster to balance everything out.
It might help to go outside after a rain and look at the water running in a curb gutter. It will be about the same depth downstream as it is upstream.
The surface of the river will generally be sloped at the same grade as the river bottom.
However, the water surface is dependent upon many things, including the cross-sectional shape of the channel & floodplain, roughness of the channel bed, bends in the river, etc. For example, the water will “pile up” at the outside of bends; it will be higher there than at the inside of the bend.
Also, when the water flows through a bridge or culvert, it is possible for the water-surface profile to slope upward instead of downward.
Yes, rivers have slope. When placed in the perspective of the surrounding topography, the overall gradient of a river gives a good indication of how difficult it will be to paddle down.
When drifting along, you usually will not notice the slope, aside from the occasional riffle or eddy. At times, however, the surface will seem to tilt as the water makes a quick descent down a steep section of river bottom.
http://www.geocities.com/Yosemite/4444/244931.jpg
What’s interesting is that as the water flows down the drop into a pool, the water in the pool says “We’re all full here! Go away!” The water flowing down into the pool is forced to shoot up into the air while waiting for the water in the pool to make room for it.
Excellent answers everyone. Those pictures helped a lot too.
Now, doesn’t water flow in horizontal sections at different speeds? I guess the whole notion that water is really made up of infinite tiny droplets, yet appears like a solid entity, is the thing that makes my brain twist.
Wu Wei of White Water by Chuang-tzu (Copyright Richard Culpeper)
The great waterfall of Lu Canyon is thousands of feet high, with a halo of mist that can be seen for many miles. Nothing survives the violent waters at its foot. Yet once K’ung Fu-tzu saw an old man swim the tempest. K’ung-Fu-tzu and his retainers ran with ropes to rescue him, but when they descended to the floor of the canyon they found the man sitting on a large boulder, quietly singing.
K’ung Fu-tzu exclaimed, “You cannot be alive! What are your powers to allow you to do what you have done?” The old man turned and smiled, “I am just a man, but I began to learn as a boy, and I continue to practise. I flow with the water, going up, down and around with it. I forget myself and do not struggle against forces far beyond my control. Then I use my meagre abilities in the moments when the water and I share the same path.”
Imagine you are at Water World, at the Giant Slide. You have brought with you a bazillion tiny cubes of red Jello (each about the size of a sugar cube).
Go to the top and pour the cubes down the slide. Imagine how they would look, all stacked on top of each other: the cubes on the very bottom feel the weight of all the others on top.
What happens when you push something slippery? It skeets away from you.
So, as the cubes are sliding down the slide, the ones on the bottom slide the fastest - to get away from the pushing of the others - the next layer a little less fast, the next layer, and so on, until the very top layer of cubes is sliding the most slowly.
This is why you can’t judge the bottom current of a river by the speed of things floating on the top.
At places where the slide bends, the cubes going around the inside of the curve go faster - because they cover less distance - just like race cars.
If God reached down and squeezed the Giant Slide to make a bottleneck, the cubes would squirt out of the downstream side, just like water from your hose when you put your thumb over it.
I would add that there is also frictional resistance along the river bed, so the bottom water is not the fastest. Rather, the fastest is a tube in the middle of the flow, like the copper in the middle of an insulated wire.
Jim Snyder, who developed the sport of squirt boating (underwater wild water kayaking) explains the flow as follows:
If you Google “Squirt” and “Brain Douche” you will find a number of sites that get into mystery moves in which kaykers descend to the tube and ride along it underwater for a while.
The explaination between the uneven bottom and the surface of the river is because: Water seeks its own level. If the level of the water is lower than a part of the bottom then you have an island.
Just to be sure we’re clear, when a river meanders, the water on the outside edge, passing the cut bank, of the curve travels faster, as it has farther to go in about the same amount of time as the water passing the inside bank. The water on the inside of the curve will usually slow down, and as such it will lose some ability to continue transporting its sediment load, and sediment will be deposited. That’s how point bars are formed on the inside curve of river meanders.
The fastest water drop would be the slipperiest. And that would be in the innermost stream tube … not always in the geometric center of the cross-section, but the centroid, if you will, of the field of resistance caused by the bed forms, gravity, shear stresses, etc. … just like the kayaker says.
BTW what a rush that would be. I mean, I think I would die of happiness to experience that. Underwater wild water kayaking, I mean.
Yes, Ringo, my race-car analogy was flawed. But not in the manner of your comment. I should have said, “they get where they are going faster” not “they go faster”.
'round here, most of our streams & rivers deposit sand on the outside of the bend. The current is strongest on the inside of the bend. This is not necessarily going to be true for any given stream on the planet.
To accurately predict the location of the strongest current in any given stream, one would scale-model it, like the Corps of Engineers does.
According to their Hydrologic Engineering Center publication “Sediment Transport” (chapter 3), fluvial morphology is a complex process which is poorly understood. The locations where rivers either take up bed material or deposit it can shift with time. These shifts are dependent upon hydraulic controls and availibility of bed load material.
I reckon I was thinking of the bend as a control section, where the jello cubes would shoot out after they passed through it, like the water does when you put your thumb over your garden hose. I’ts just hard to explain a complex thing like river mechanics with a simple analogy and get every part covered at once.
I can’t believe I’m thinking this hard on Sunday. I need to get a life
Actually, it’s not as simple as I described above. Where point bar deposits occur there is a helicoidal flow of surface water towards the (outer) cut bank and a bottom flow towards the inner bank where sediment is released. Braided rivers are different.
