To my understanding, a river’s source is fueled by snow and rain, but is it enough to keep it running for that long? Afterall the source doesn’t get rain/snow 24/7 so wouldn’t bigger rivers drain the source in a matter of weeks instead of many hundreds of years?
In: Planetary Science
Most rivers will vary dramatically over the course of years in terms of how much water is flowing in them. Sometimes they might get nearly dry or even completely dry, and at other times they might overflow their banks.
One thing to keep in mind is that the presence of lakes helps to normalize the level of rivers. They typically form where there is a bottle neck holding back much of the potential flow of the river and only allowing a smaller amount out. The rest backs up and forms a reservoir, and when there is not as much rain or snow it will continue to trickle out from that reservoir for a long time.
Bigger rivers are FED by larger areas.
For example, the Mississippi River? The giant river down the middle of the USA, it’s basin covers over 1/3rd of the USA (and extends slightly into Canada)
Meaning ALL the rain from Pittsburgh Pennsylvania all the way to Colorado and Montana will eventually drain down through the Mississippi.
And that draining takes time. It doesn’t just instantly rush into the rivers, there are tiny creeks, streams, smaller tributaries that gradually bring the water in. And then there’s also MILLIONS of ponds and lakes and beaver dams that slow the flow of the water down while it gets there.
Plus, a lot of the water doesn’t even go straight into the creeks and streams, it lands on the dirt, is absorbed by the dirt/ground, and then gradually over weeks or months flows through the soil itself. Which also slows it way down.
And yes, during times of drought even giant rivers can get shallower, and during very wet times they can overflow their normal riverbanks.
But generally speaking, there’s so much water over such a wide area of land that it all is just gradually making its way down to the Mississippi, and eventually the Gulf of Mexico. Keeping the river fed.
Precipitation (rain, snow, etc) delivers water uphill, and the water runs downhill. Sometimes a big rain delivers water faster, other times lack of rain causes less water to be moved. This corresponds to water levels going up and down over time, as we get more or less water flowing.
Yes, the precipitation carries enough water to keep things moving. Consider that all of the water coming down in all of the square miles of land upstream has to flow out through that stream.
Rivers vary – sometimes they have strong flow, sometimes weak, sometimes they dry up. Sometimes, they freeze in the winter. Sometimes the pieces of the river disconnect to form shallow ponds that will later reconnect when there’s more water. The strength of the flow and the width of the river is going to depend on how much water is flowing at the time.
Rivers aren’t that much higher than the bays or oceans they drain into. The river is fresh, but then there’s a brackish area that’s a mix of river water and salty ocean water.
When a river bed “dries up” it’s not just about draining all the water to the ocean. Some sinks into the ground. Some goes into smaller ponds.
Human interventions around the Nile river, the Mississippi river, and others use levees and flood walls so that people can live close to the river year round without getting flooded out when the river is having a particularly wet season.
|is it enough to keep it running for that long?
Demonstrably, yes, it is.
Rivers don’t just have one source. They have tributaries, which can gather rain water from enormous amounts of land. That’s why rivers start out small, and get larger and larger as they approach the ocean.
Consider [this image of the Mississippi basin](https://www.mvd.usace.army.mil/portals/52/siteimages/P1_new.jpg). That river is gathering rain from almost half of America.
The [French Broad River](https://en.wikipedia.org/wiki/French_Broad_River?wprov=sfti1#History) flows east to west *across* the Appalachian Mountains, because it is *older* than the Appalachian Mountains.
It is a river that is older than North America. Older than land-based life more complex than microorganisms. Older than *plants*. It has almost no fossils in it because *it is older than fossils*.
Rivers can be really, really, really old.
The basin that a river draws from is often quite large. The most extreme example is the mississippi, which drains from about 20 states.
It is all about the fact that rivers drain from incredibly large areas of land.
That water can run down. Look at the colorado river which has had extreme drought for decades and the total flow from the river has been steadily declining over time.
When it rains, the water gets absorbed into the soil. It can stay in the soil for up to hundreds of years, slowly traveling downhill until some of it emerges at the bottom of the valley to form a river. So there’s plenty of water in the soil to keep the river flowing until the next rain.
And when there is too much rain for too long to get absorbed into the soil, it stays on the surface and that is called a flood.
It seems mind blowing that there’s enough rain falling in the river catchment to keep the enormous river flowing constantly, but there has to be, the water has no other way of going uphill.
Let’s say the total amount of water in all the rivers in the world is *r* liters.
Icy Mountains have 4*r* liters. (Note: I’m not including Antarctic, Greenland and Arctic Islands because those places have very low demographic density and are not fueling rivers for like 99% of humankind).
Swamps have 6*r* liters.
The atmosphere have 7*r* liters.
The fresh water lakes have 45*r* liters.
The non-saline groundwater have 5000*r* liters.
Everything above is what ends up sending water to rivers. It is easy to see that the rivers don’t dry as the water fueling those rivers is in many orders of magnitude higher. A river loses water to either the soil or the air, both which will end up using this water to refuel the river.
How is reddit now doing the job of a literal elementary school teacher? What the hell is happening in schools these days?
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