英語聽力文摘 下沉的河流三角洲

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Sinking River Deltas

據(jù)《生活科學(xué)》網(wǎng)站報(bào)道，美國科學(xué)家最新的研究報(bào)告顯示，由于人類活動(dòng)的影響，世界上大部分的河流三角洲正在不斷下沉，使得該地區(qū)更易遭受到洪水和海洋風(fēng)暴的襲擊，數(shù)千萬人的生存正面臨著威脅。美國的新奧爾良正是一個(gè)處于下沉三角洲的城市，它遭受卡特里娜颶風(fēng)的蹂躪的例子就是一個(gè)活生生的證明。研究人員根據(jù)NASA人造衛(wèi)星影像，對(duì)各大三角洲的地區(qū)進(jìn)行了橫向和縱向的比對(duì)，得出了主要的三角洲正在不斷下沉的結(jié)論。報(bào)告把各三角洲地區(qū)受威脅的程度分為五個(gè)級(jí)別，亞洲有11個(gè)地區(qū)被列入受威脅最嚴(yán)重的級(jí)別，中國占了其中3個(gè)，它們是黃河三角洲、長江三角洲和珠江三角洲。此外，埃及的尼羅河三角洲、泰國的湄南河三角洲和法國的隆河三角洲也都屬于受威脅最嚴(yán)重的級(jí)別。受威脅程度略低于上述幾個(gè)三角洲的地區(qū)為：孟加拉的恒河三角洲、緬甸的伊洛瓦底江三角洲、越南的湄公河三角洲和美國的密西西比三角洲。

River deltas are the broad coastal valleys where rivers end their long trek and empty into the sea. Standing on a river delta, 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 sediment 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 sagging underneath, 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 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 sags 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.