How long is a river?

riverDoes it seem simple to measure the length of a river? Could you grab a tape measure and run it from the start of the river to the end? The trouble with this method is that rivers don’t go in straight lines – they bend.

On flat ground, rivers often curve and form wide bends and loops. All these bends make the river a lot longer than it could be if it followed the most direct path.

If there’s a slight bend in a river, then sand and sediment tends to be deposited on the inside of the curve. Meanwhile, the outside of the bend tends to become eroded. These two actions make small bends into big loops know as meanders.

A meander can balloon out to five or ten times the width of the river. However, if the loop gets too big, the start and end of the loop can join. Then the river follows the shorter path and the rest of the loop becomes a billabong.

Hans-Henrik Stølum, a scientist from the University of Cambridge, investigated the length of rivers. He used a computer model to simulate how rivers change shape over time, getting longer and shorter as meanders form then join up.

Although Hans-Henrik found the rivers got longer some times and shorter other times, they never got very long or short. The model rivers always stayed about three times longer than a straight line.

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