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TED - Why rivers curl?

Compared to the whitewater streams that tumble down mountainsides,

to tumble - кувыркаться, вваливаться, падать

the meandering rivers of the plains may seem tame and lazy.

meandering - извилистый
tame - скучный, банальный (ос. значение - ручной)

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

to corral - загонять; загонять в загон; окружать лагерь повозками
steep - крутой, отвесный
to carve - вырезать, высекать; резать, разделывать

– their courses are literally set in stone.

literally - буквально

Out on the open plains,

plain - равнина

those stony walls give way to soft soil,

soil - почва, грунт, земля

allowing rivers to shift their banks

to shift - сдвигать, перекладывать, переложить

and set their own ever-changing courses to the sea:

courses that almost never run straight.

straight - прямой

At least not for long,

at least - по крайней мере, по меньшей мере

because all it takes to turn a straight stretch of river into a bendy one

stretch - протяжение, простирание
bendy - гибкий, извилистый (to bend - гнуть, изгибать)

is a little disturbance and a lot of time

disturbance - возмущение, нарушение, беспокойство

and in nature, there’s plenty of both.

plenty - много, достаточно

Say, for example,

that a muskrat burrows herself a den in one bank of a stream.

muskrat - ондатра
to burrow - рыться, зарыться, рыть нору

Her tunnels make for a cozy home,

cozy - уютный, удобный

but they also weaken the bank,

to weaken - ослаблять, слабеть

which eventually begins to crumble and slump into the stream.

to crumble - крошиться, крошить, покрошить, раскрошить
to slump - резко падать, тяжело опускаться; сутулиться, горбиться

Water rushes into the newly-formed hollow,

hollow - лощина, пустота, дупло, полость, впадина, низина

sweeping away loose dirt and making the hollow even hollower,

to sweep - мести, сметать

which lets the water rush a little faster

and sweep away a little more dirt

and so on, and so on

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

to divert - отвлекать, отводить, отклонять

and away from the other side of the channel,

the flow there weakens and slows.

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

that dirt drops to the bottom and builds up

to build up - наращивать, строить, укреплять, застраивать

to make the water there shallower and slower,

shallow - мелкий, мелководный, поверхностный

and then keeps accumulating until it becomes new land on the inside bank.

Meanwhile, the fast-moving water near the outside bank

sweeps out of the curve with enough momentum

momentum - импульс, момент, инерция

to carry it across the channel and slam it into the other side,

to slam - хлопать, захлопнуть, прихлопнуть

where it starts to carve another curve.

And then another, and then another, and then another.

The wider the stream,

the longer it takes the slingshotting current to reach the other side,

and the greater the downstream distance to the next curve.

downstream - вниз по течению

In fact, measurements of meandering streams all over the world

measurement - измерение, обмер

reveal a strikingly regular pattern:

strikingly - поразительно

the length of one S-shaped meander

tends to be about six times the width of the channel.

to tend - иметь тенденцию, иметь склонность (к чему то)

So little tiny meandering streams

tend to look just like miniature versions of their bigger relatives.

relative - родственник

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

its curves will continue to grow curvier and curvier

until they loop around and bumble into themselves.

to bumble - шлепаться; заикаться, путать, запинаться

When that happens,

the river's channel follows the straighter path downhill,

downhill - спуск, скоростной спуск

leaving behind a crescent-shaped remnant called an oxbow lake.

remnant - остаток, пережиток, след

Or a billabong. Or a lago en herradura. Or a bras mort ...

We have lots of names for these lakes,

since they can occur pretty much anywhere liquid flows or used to

to occur - происходить, иметь место
liquid - жидкость

– which brings up an interesting question:

what do the Martians call them?