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One river bend at three stages, growing until it is cut off Three framed maps stand side by side, each showing the same stretch of river from above. In the first the channel crosses the map with one shallow bend in it. In the second that bend has swollen into a deep horseshoe loop that bulges out much wider than the narrow strip of land between its two ends. In the third the channel runs almost straight across the map again, and the loop it used to take is left above it as a separate closed horseshoe of water, joined to the river at neither end.

A River Bend Can Only Get Bigger

About 14 minutes

Look at a river from above — in a photograph taken from a plane, or on a map — and it wanders. It loops one way, then swings back the other, then loops again, like a piece of loose string dropped on the floor.

That should be strange. Water is being pulled straight downhill by gravity. Nothing is steering it. A ball rolling down the same slope would go more or less straight.

And there is a second thing, stranger than the first, which is what this article is really about. Rivers do not wander at random. A bend only ever grows. Watch the same stretch for fifty years and every curve is deeper than it was. Not one of them quietly flattens back out.

The outside of a bend is in a hurry

Put your finger on the inside of a bend and trace it round. Now do the outside. The outside is a longer journey — it has further to go to reach the same place downstream.

So the water shoves. As the river comes into a curve it cannot turn on the spot, and the fastest thread of water swings wide and piles against the outer bank. The inside, meanwhile, is a quiet corner. Water there is slow, shallow and hardly moving.

A river bend seen from above and sliced across On the left, a bend in a river is drawn as seen from above, as a curved channel between an outer bank and an inner bank. A long arrow along the outside of the curve is labelled fast, and a much shorter arrow along the inside is labelled slow. A dashed line marked A to B crosses the channel at the tightest part of the bend. On the right, that same A to B line is drawn as a slice through the river. The bed shelves gently down from the inner bank, where short marks show sand piling up, then drops to the deepest water hard against the outer bank, which rises as a steep wall standing above the waterline. fast carries the bank away slow drops its sand A B from above A B sand arriving deepest here, and cutting the bank sliced across
The same bend, looked at from above and then sliced across. Deep and fast against the outer bank. Shallow and slow on the inside.

That difference in speed is the whole story, because fast water carries and slow water drops. A river is not clear — it is full of sand and grit it picked up upstream. Speed is what keeps that load in the air. Take the speed away and the load falls out.

So on the outside, fast water tears at the bank and carries the pieces off. On the inside, slow water quietly puts sand down. There is even a sideways roll to it: the water going round a bend turns over as it goes, sweeping along the bed and pushing sand across to the inside.

One bank is being eaten. The other is being built.

Which is why the bend has to grow

Now follow what that does over time.

The outer bank moves outward, because material keeps leaving it. The inner bank moves outward too, because material keeps arriving on it. Both banks travel the same direction at roughly the same rate, so the river stays about the same width — it just slides sideways, further into the curve.

And here is the trap the river cannot get out of. A deeper bend has a tighter outside. A tighter outside throws the water harder against the bank. Harder water cuts faster.

The bend makes itself worse. Each year of cutting sets up a slightly sharper corner for next year's water to attack.

One river bend sweeping further outward as the years pass A river runs across the picture from left to right with a bend arching upward in the middle of it. As the years are added, further arches are drawn above the first, each springing from the same river but reaching higher and swinging wider, so the set of them records the path the bend has taken as it moved. Every new arch also starts and finishes slightly closer to the middle than the one before, so the strip of land between the two ends of the loop, marked below as the neck, gets narrower at every stage. Nothing at any stage becomes straighter. the neck, and it is still getting thinner each new line is where the channel had moved to Nothing on this slider ever makes the river straighter.

Drag the years forward and watch one bend do the only thing it knows how to do.

Nothing on that slider straightens. The loop swings further out, the two ends crowd closer together, and the strip of land between them — the neck — gets thinner and thinner.

The only escape is to abandon the loop

Eventually the neck is barely there. Then comes a big flood, the river tops its banks, and the water takes the short way across instead of going all the way round.

Once it does that even for a day, it has found a much steeper, much faster route — and it never gives it back. The new channel deepens. The old loop is left off to one side with no current running through it, and silt seals up both ends. What is left is a curved lake shaped exactly like the bend that made it. Geographers call it an oxbow lake, and the name comes from the U-shaped collar that used to go round an ox's neck.

Notice what did not happen there. The river did not un-bend. The bend is still sitting in the field — it just has no river in it any more. The channel abandoned the loop rather than reversing it, which is a different thing, and it is the only way out a river has.

A river cuts across the neck of a loop during a flood. What has happened to the bend?

What to look for

You do not need a plane. Find any stream with a bend in it and stand at the outside of the curve.

The bank under your feet will be steep, often cut away underneath, sometimes with tree roots hanging out of it in mid-air. Now look straight across. The far bank will be low, and there will be a slope of clean sand or gravel running gently down into the water.

Those two banks are the same age and the same river. One of them is being taken away, one grain at a time, and dropped on the other.