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Compared to the whitewater streams that tumble
down mountainsides, the meandering rivers

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of the plains may seem tame and lazy.

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But mountain streams are corralled by the
steep-walled valleys they carve – their

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courses are literally set in stone.

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Out on the open plains, those stony walls
give way to soft soil, allowing rivers much

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more freedom to shift their banks and set
their own ever-changing courses to the sea:

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courses that almost never run straight.

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00:00:26,838 --> 00:00:30,960
At least not for long, because all it takes
to turn a straight stretch of river into a

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bendy one is a little disturbance and a lot
of time  – and in nature, there’s plenty

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of both.

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Say, for example, that a muskrat  burrows

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herself a den in one bank of a stream.

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Her tunnels make for a cozy home,  but they
also weaken the bank, which eventually begins

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to crumble and slump into the stream.

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00:00:48,659 --> 00:00:52,738
Water rushes into the newly-formed hollow,
sweeping away loose dirt and making the hollow

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00:00:52,738 --> 00:00:57,078
even hollower, which lets the water rush a
little faster and sweep away a little more

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00:00:57,079 --> 00:00:59,730
dirt from the bank...and so on, and so on
.

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As more of the stream’s flow is diverted
into the deepening hole on one bank and away

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from the other side of the channel, the flow
there weakens and slows.

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And since slow-moving water can’t carry
the sand-sized particles that fast-moving

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water can, that dirt drops to the bottom and
builds up to make the water there shallower

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00:01:17,140 --> 00:01:21,079
and slower, and then keeps accumulating until
the edge of the stream becomes new land on

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the inside bank.

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Meanwhile, the fast-moving water near the

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00:01:24,530 --> 00:01:29,390
outside bank sweeps out of the curve with
enough momentum to carry it across the channel

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and slam it into the other side, where it
starts to carve another curve . And then another,

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and then another, and then another.

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The wider the stream, the longer it takes
the slingshotting current to reach the other

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side, and the greater the downstream distance
to the next curve.

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In fact, measurements of meandering streams
all over the world reveal a strikingly regular

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pattern : the length of one S-shaped meander
tends to be about six times the width of the

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channel . So little tiny meandering streams
tend to look just like miniature versions

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of their bigger relatives.

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As long as nothing gets in the way of a river’s

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meandering , its curves will continue to grow
curvier and curvier until they loop around

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and bumble into themselves.

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00:02:08,330 --> 00:02:14,250
When that happens, the river follows the straighter
path downhill, leaving behind a crescent-shaped

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remnant called an oxbow lake.

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00:02:16,319 --> 00:02:17,319
Or a billabong.

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Or a lago en herradura.

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Or a bras mort ...
    

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We have lots of names for these lakes, since
they can occur pretty much anywhere liquid

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flows – which brings up an interesting question:
what do the Martians call them?

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