Where did rivers come from?
If it weren’t for rivers, there would be no civilization. But where did they come from why do they flow somewhere for thousands of years without drying up, and, most of all, why don’t they stop wriggling and just flow in a straight line?!
Why rivers flow
Water appeared on the surface of the Earth both from its own «reserves» left in the interior after the planet’s formation, and from outside—from collisions with comets and other icy celestial bodies (yes, the water from your tap may well be of alien origin).
The first rivers seem to have appeared along with the first rain more than 3.5 billion years ago, as soon as the planet cooled enough for liquid water to exist. Rain and other precipitation are the primary sources of water in rivers to this day.
By definition, rivers are a natural flow of freshwater that gravity forces to move. Obeying gravity, a river, like a teardrop, simply flows down the path of least resistance, and ends up either drying up along the way or flowing into a larger body of water—a lake, a sea, or an ocean.
The delta of the Siberian Lena River flowing into the Arctic Ocean. Photo: USGS / Unsplash
Often, a lake at the end of a river appears precisely because of the river itself, which rests in a natural hollow in the landscape, and fills it like a bowl. Over time, the lake stops expanding, due to evaporation and water escaping into the ground.
Other sources of river water besides rain include:
Melting glaciers
Snow caps
Lakes
Groundwater
Rainfall also replenishes groundwater. Thanks to this recharge, many rivers don’t dry up entirely, even during droughts.
Streams that seem to come out of nowhere, and may eventually flow into larger rivers, are often just a consequence of the same rain—somewhere, the soil absorbs so much rainfall that water rushes to the surface.
The water cycle in rivers is a closed circuit, as you can easily guess, starting and ending with rainfall: water in rivers evaporates, vapor forms clouds, clouds shed rain, and so on. Well, you get the idea.
The formula for river formation is simple: precipitation must be greater than water evaporation and absorption into the soil. Only then does the excess water form trickle down to form rivers.
Why rivers bend
It could seem that the easiest way to flow is in a straight line. But for some reason, rivers are always bending. And it’s not just the peculiarities of the landscape. Moreover, the older the river, the deeper its bends can be. Why does this happen?
Aerial shot of the Okavango River. Photo: Wynand Uys / Unsplash
It starts with the different speeds at which the water flows. In the center of the stream, the water flows faster, and closer to the banks and the bottom, the flow is slower because the water creates friction against the ground. At the same time, the water erodes the banks and leaves its sediment—sand and rocks—on them.
Since the banks have irregularities (trees, rocks, different depths) and may be made up of rocks of different hardness, the flow velocity may differ from one bank to another. Because of this, more sediment will accumulate on one bank, while on the other bank, the land will be more eroded. This is how a river bend is created.
The bend deepens because the center and fastest part of the water presses on it. The river tends to flow straight ahead by inertia, but now it presses partially against the bank, eroding it.
The left picture shows the river trying to flow straight. In the center picture, a fallen tree causes sediment to build up on the right bank, while the flow is diverted to the left bank. In the right picture, the stream has eroded the left bank further, while the sediment on the right bank is overgrown with vegetation.
The process of deepening the bend never stops; it only slows down due to natural conditions. For example, hard rock may be exposed on the eroding bank, or the river flow itself may slow down because the bend is too tight.
It also happens that bends (meanders) are so deep that they create rings (old rivers) and then make their way between the rings. Meanders of the Amazon River. Photo: Ivars Utināns / Unsplash
What can happen next
As you have already guessed, rivers are not passive subjects of nature, meekly adjusting to external conditions, no matter how it may seem at first glance. On the contrary, like glaciers, rivers can change the face of the planet.
Deep canyons and mountain valleys are often the result of millennia and millions of years of river flow.
The Grand Canyon in the United States, maximum depth of 1829 meters, was literally carved out of stone by the flow of the Colorado River. Photo: Michael Kirsh / Unsplash
For people, rivers can be dangerous when the flow is increased by heavy rains or snowmelt runoff, causing the river to overflow its banks. However, for plants and animals, on the contrary, this is mostly a positive process: a river that has burst its banks fills everything around it with nutrients.
We can say that rivers edited the planet’s landscape for billions of years before us, and will continue for at least another billion until the Sun evaporates all of them—or climate change, if we’re not careful.
Text: Jason Bright, a journalist, and a traveler
Cover photo: John Salvino / Unsplash
Read more:
Human survival at great depths
Explore the different types of precipitation. Rain and snow are just the beginning
