【英语时差8,16】沉河三角洲

River deltas² are the broad coastal³ valleys where rivers end their long trek⁴ and empty into the sea.   Standing⁵ on a river delta¹, you might expect to see rushing water, but you wouldn't expect the ground beneath your feet to be rushing anywhere. Actually, that ground is probably rushing straight down. Most major river deltas are sinking.

What happens is this: Rivers wash away dirt and sediment⁶ from the lands they pass over. Much of this ends up at the river's delta, where the waters spread out and slow down before joining the sea. Over time, sediment can pile so heavily it can actually cause the Earth's crust to start sagging⁷ underneath⁸, maybe an inch or so downward every decade. What's more, as this sediment landscape dries out it compresses, sinking even more. Those muddy waters of the Mississippi have left behind such a weight of mud that New Orleans is currently below sea level. If river deltas sink, what keeps them from going under? After all, the sea is right there, waiting to flood any land that sinks too low.

The answer is all that sediment. Although the Earth's crust sags⁹ under its weight, the river always has new sediment to deposit--especially when the delta is allowed to flood periodically. This rising sediment keeps pace with the sinking crust, and the level of the land stays pretty constant.

Sinking deltas become a problem if the river's waters are dammed or channeled, rushed to the sea without a chance to spread new sediment over the wide, delta valley. This lack of new sediment is what causes New Orleans to be below sea level.  

1 delta   

n.（流的）角洲

参考例句：

• He has been to the delta of the Nile.他曾去过尼罗河三角洲。
• The Nile divides at its mouth and forms a delta.尼罗河在河口分岔，形成了一个三角洲。

2 deltas   

希腊字母表中第四个字母( delta的名词复数 )； （河口的）三角洲

参考例句：

• Tidal channels, deltas, and washover fans are characteristically associated with offshore bars. 潮汐水道、三角洲和冲刷扇典型地与滨外砂洲伴生在一起。
• I know many of the early civilizations prospered on deltas. 我知道很多古老的文明都是在三角洲上蓬勃发展起来的。

3 coastal   

adj.海岸的，沿海的，沿岸的

参考例句：

• The ocean waves are slowly eating away the coastal rocks.大海的波浪慢慢地侵蚀着岸边的岩石。
• This country will fortify the coastal areas.该国将加强沿海地区的防御。

4 trek   

vi.作长途艰辛的旅行；n.长途艰苦的旅行

参考例句：

• We often go pony-trek in the summer.夏季我们经常骑马旅行。
• It took us the whole day to trek across the rocky terrain.我们花了一整天的时间艰难地穿过那片遍布岩石的地带。

5 standing   

n.持续，地位；adj.永久的，不动的，直立的，不流动的

参考例句：

• After the earthquake only a few houses were left standing.地震过后只有几幢房屋还立着。
• They're standing out against any change in the law.他们坚决反对对法律做任何修改。

6 sediment   

n.沉淀，沉渣，沉积(物)

参考例句：

• The sediment settled and the water was clear.杂质沉淀后，水变清了。
• Sediment begins to choke the channel's opening.沉积物开始淤塞河道口。

7 sagging   

下垂[沉，陷]，松垂，垂度

参考例句：

• The morale of the enemy troops is continuously sagging. 敌军的士气不断低落。
• We are sagging south. 我们的船正离开航线向南漂流。

8 underneath   

adj.在...下面，在...底下；adv.在下面

参考例句：

• Working underneath the car is always a messy job.在汽车底下工作是件脏活。
• She wore a coat with a dress underneath.她穿着一件大衣，里面套着一条连衣裙。

9 sags   

向下凹或中间下陷( sag的第三人称单数 )； 松弛或不整齐地悬着

参考例句：

• The bed sags in the middle, and is uncomfortable. 床的中间往下塌，很不舒服。
• He sags his pants; doo rags and a stockin cap. 他穿着松弛的裤子。抹布一样的帽子。