百科知識視頻之煎蛋小學(xué)堂73 九曲河流十八彎

Compared to the whitewater streams that tumble down mountainsides,the meandering rivers of the plains may seem tame and lazy.But mountain streams are corralled by the steep-walled valleys they carve.their course are literally set in stone.Out on the open plains,those stony walls give way to soft soil,allowing rivers to shift their banks and set their own ever-changing courses to the sea:courses that almost never run straight.

同山頂飛流直下濺出銀白水花的湍流相比，平原上蜿蜒的河流就太無趣了點。不過 山間的湍流被困于它們蝕刻出的深谷。水流游走于巨石之間。到了開闊的平原 石壁被柔軟的土壤取代，河岸地勢逐漸提高 并由此奔向大海：但奔流時從不走直路。

At least not for long.because all it takes to turn a straight stretch of river into a bendy one is a little disturbance and a lot of time.And in nature,there's plenty of both.Say,for example,that a muskrat burrows herself a den in one bank of a stream.Her tunnels make for a cozy home,but they also weaken the bank,which eventually begins to crumble and slump into the stream.

即使有時間也不長，因為 如果想把直流扳彎只需細微的干擾和極長的時間。自然中 這二者可不缺。比如 一只麝鼠在河岸某側(cè)打出一個洞。這地道是為溫暖小窩造的 但削弱了河岸，使其破裂并落入水流中。

Water rushes into the newly-formed hollow,sweeping away loose dirt and making the hollow even hollower,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 and away from the other side of the channel,the flow there weakens and slows.

水流沖入剛形成的洞中，帶走松散的泥土 讓洞更深，這也使水流流速加快 帶走更多的泥土 周而復(fù)始 由于有深坑一側(cè)的河水流量更多 在河的另一側(cè) 水流流速有所降低。

And since slow-moving water can't carry the sand-sized particles that fast-moving water can,The dirt drops to the bottom and builds up to make the water there shallower and slower,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 to carry it across the channel and slam it into the other side,where it starts to carve another curve.

由于緩流攜帶砂礫的能力不及湍流，泥土便落在水底 逐漸堆積 使該處河水更淺 流速更慢，泥土繼續(xù)積聚 直到內(nèi)岸變成新的土地。與此同時 外岸處的湍流 沖出彎曲處 帶著足夠的沖力橫穿河道 撞在河岸另一側(cè) 并開始割出另一道弧。

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.In fact,measurements of meandering streams all over the world reveal a strikingly regular pattern:the length of one S-shaped meander tends to be about 6 times the width of the channel.

一道又一道 一處接一處。河流越寬 彈弓般的水流到達岸的時間就越長，弧間沿流距離也越長。實際上 對全世界蜿蜒河流的測量結(jié)果 展現(xiàn)了一個驚人的規(guī)律：S形曲流的長度大約為河道寬度的6倍。

So little tiny meandering streams tend to look just like miniature versions of their bigger relatives.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.When that happens,the river's channel follows the straighter path downhill,leaving behind a crescent-shaped remnant called an oxbow lake.

所以 小彎流看上去正像是大彎的“迷你版”。若河流彎曲時不受外界阻礙，其彎曲處的弧度會越來越大直到形成環(huán)繞回自己身上，此時 河道會沿一條直路流下山，留下新月狀的“牛軛湖”。

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

或說“死河”(澳洲說法)“馬蹄湖”(西語)“回水”(法語)……這種湖的叫法很多，在水流曾過的地方都可能發(fā)生。這也引出了另一個奇妙的問題：火星人該怎么說呢?