A purely scientific explanation for why every river on Earth meanders.
If we exclude artificial rivers and lakes, it’s safe to say that no river system in the world flows in a straight line. If you don’t believe it, you can verify this yourself using Google Earth.
A straight line is the shortest path, yet rivers seem to “make things difficult” for themselves, continuously winding and stretching for thousands of kilometers. The question here is: Why?
Rivers Take Time to Curve
The curvature of rivers results from a combination of many natural factors. Water always flows downhill, but that downhill path might not be straight. And even if the
water’s path starts straight, over time, the river will still become curved. The degree of a river’s curve comes from a combination of many natural factors.
Imagine this: suddenly, an otter “excavates” one side of the riverbank to build a nest. It’s a small hole, but it inadvertently weakens the soil on that bank. Water will gradually flow into the hole at an increasingly faster rate as the soil erodes, while simultaneously weakening the water flow on the opposite bank. That slower current carries sediment, depositing it on the other bank. And over time, the straight section of the river literally becomes curved.
Of course, the otter is just a playful example. In reality, there are many natural factors at play that are sufficient to curve a river without needing an otter to dig.
From a scientific perspective, rivers flow not in straight lines but meander due to two main reasons: topography and the Coriolis force.
The reason of topography is probably easy to understand. Rivers tend to flow from higher elevations to lower elevations; in areas with rugged terrain, the river will meander.
The Coriolis force is a consequence of the Earth’s rotation on its axis. As the Earth rotates, every point at different latitudes on its surface (except the two poles) has a different tangential velocity and moves from West to East. Therefore, objects moving on Earth will be deflected from their initial direction. The Coriolis force strongly affects the direction of movement of air masses, ocean currents, rivers, ballistic trajectories, etc. This is why rivers meander, with erosion on one side and deposition on the other.
At the beginning of their formation, riverbeds are usually not flat. As river water flows, for various reasons, the flow speed on the left and right sides is not completely equal. Here, the riverbank erodes a bit; there, a tree is lost; elsewhere, more water flows in from outside… These phenomena can all cause the river’s flow speed to increase or decrease in certain places. Simultaneously, the material on the two banks also differs; some areas are easily eroded, while others are quite solid. All of these factors cause the riverbed to become winding and tortuous.
Bending a section of a river means changing the direction of the water flow. The redirected water flow creates a force acting on the riverbank soil, increasing erosion, and then the rest of the river also gradually curves as an inevitable law.
The larger the discharge of a river section, the greater the distance between its meanders. But there’s an interesting point common to all rivers in the world: the length of an S-shaped meander loop is approximately 6 times the width of the river. This means a short river section will resemble a miniature version of a larger river bend.
This curving process occurs continuously over time. And if it doesn’t encounter obstacles (like boulders, mountains…), at some point, two meanders of the river will meet. At that moment, the river will flow straight again, leaving behind its “curved” past. This “past” is called an oxbow lake.
An oxbow lake can be said to be a characteristic feature indicating the existence of a nearby river. And did you know, there are also traces of oxbow lakes on Mars, suggesting that the Red Planet once contained many oceans and a rich river system.
