英语听力文摘 下沉的河流三角洲
Sinking River Deltas
据《生活科学》网站报道，美国科学家最新的研究报告显示，由于人类活动的影响，世界上大部分的河流三角洲正在不断下沉，使得该地区更易遭受到洪水和海洋风暴的袭击，数千万人的生存正面临着威胁。美国的新奥尔良正是一个处于下沉三角洲的城市，它遭受卡特里娜飓风的蹂躏的例子就是一个活生生的证明。研究人员根据NASA人造卫星影像，对各大三角洲的地区进行了横向和纵向的比对，得出了主要的三角洲正在不断下沉的结论。报告把各三角洲地区受威胁的程度分为五个级别，亚洲有11个地区被列入受威胁最严重的级别，中国占了其中3个，它们是黄河三角洲、长江三角洲和珠江三角洲。此外，埃及的尼罗河三角洲、泰国的湄南河三角洲和法国的隆河三角洲也都属于受威胁最严重的级别。受威胁程度略低于上述几个三角洲的地区为：孟加拉的恒河三角洲、缅甸的伊洛瓦底江三角洲、越南的湄公河三角洲和美国的密西西比三角洲。
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.
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