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Why Do Rivers Curve
Compared to the whitewater streams that tumbledown mountainsides, the meandering rivers of the plains may seem tame and lazy. Butmountain streams are corralled by the steepwalled valleys they carve â€“ their courses are literallyset in stone. Out on the open plains, those stony walls give way to soft soil, allowingrivers much more freedom to shift their banks and set their own everchanging courses tothe sea: courses that almost never run straight. At least not for long, because all it takesto turn a straight stretch of river into a bendy one is a little disturbance and a lotof time Â â€“ and in nature, there's plenty of both.Â Â Â Â Â
Say, for example, that a muskrat Â burrowsherself a den in one bank of a stream. Her tunnels make for a cozy home, Â but they alsoweaken the bank, which eventually begins to crumble and slump into the stream. Water rushes into the newlyformed hollow,sweeping away loose dirt and making the hollow even hollower, which lets the water rush alittle faster and sweep away a little more dirt from the bank.and so on, and so on. As more of the stream's flow is divertedinto the deepening hole on one bank and away from the other side of the channel, the flowthere weakens and slows. And since slowmoving
water can't carry the sandsized particlesthat fastmoving water can, that dirt drops to the bottom and builds up to make the waterthere shallower and slower, and then keeps accumulating until the edge of the streambecomes new land on the inside bank. Â Â Â Â Â Â Â Meanwhile, the fastmoving water near the outside bank sweeps out of the curve withenough momentum to carry it across the channel and slam it into the other side, where itstarts to carve another curve . And then another, and then another, and then another. The widerthe stream, the longer it takes the slingshotting current to reach the other side, and the greaterthe downstream distance to the next curve.
In fact, measurements of meandering streamsall over the world reveal a strikingly regular pattern : the length of one Sshaped meandertends to be about six times the width of the channel . So little tiny meandering streamstend to look just like miniature versions of their bigger relatives.Â Â Â Â As long as nothing gets in the way of a river'smeandering , its curves will continue to grow curvier and curvier until they loop aroundand bumble into themselves. When that happens, the river follows the straighter path downhill,leaving behind a crescentshaped remnant called an oxbow lake. Or a billabong. Or a lago enherradura. Or a bras mort .
Â Â Â Â We have lots of names for these lakes, since they can occur pretty much anywhere liquidflows â€“ which brings up an interesting question: what do the Martians call themé.