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  3. Grand Canyon How was the Grand Canyon formed? The truth is that no one knows for sure though there are some pretty good guesses,

Grand Canyon How was the Grand Canyon formed? The truth is that no one knows for sure though there are some pretty good guesses,

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问题                         Grand Canyon
How was the Grand Canyon formed? The truth is that no one knows for sure though there are some pretty good guesses, and it is usually thought that a number of processes combined to create the views that you see in today’s Grand Canyon. The most powerful force to have an impact on the Grand Canyon is erosion, primarily by water(and ice)and secondly by wind. Other forces that contributed to the Canyon’s formation are the course of the Colorado River itself, vulcanism, continental drift and slight variations in the earth’s orbit which in turn causes variations in seasons and climate.
    Water seems to have had the most impact basically because our planet has lots of it and it is always on the move. Many people cannot understand how water can have such a profound impact considering that the Canyon is basically located in a desert. This is one of the biggest reasons that water has such a big impact here. Because the soil in the Grand Canyon is baked by the sun it tends to become very hard and cannot absorb water when the rains come. When it does rain the water tends to come down in torrents which only add to the problem. The plants that grow in the Grand Canyon tend to have very shallow root systems so that they can grab as much water as possible on those rare occasions when it does rain. Unfortunately these root systems do nothing to deter erosion by holding the soil in place. Now you’ve got lots of water, no place for it to go, but down to the Colorado River, and nothing holding the soil and rock in place. The result is frequently a flash flood roaring down a side canyon that can move boulders the size of automobiles, buses and even small houses. If automobiles, buses and small houses are in the way then it will take them too.(A) Luckily no one builds houses in the Grand Canyon so that’s not a problem but there are a few autos, vans and buses sitting at the bottom of the Colorado River. This mass that moves down a side canyon during a flash flood is more like fast flowing concrete than water and it can be very dangerous. You should always be well informed of weather conditions when you are hiking through side canyons in the Grand Canyon.
     (B)In the colder months, especially on the north rim, water seeps into cracks between the rocks. These cracks can be caused by seismic activity, or by the constant soaking and drying of the rocks.(C)When the water freezes it expands and pushes the rocks apart and widens the cracks. Eventually rocks near the rim are pushed off the edge and fall into the side canyons. These rocks sometimes hit other rocks and are stopped but on occasion one fall by a large rock will cause a cascading effect and create a rock fall that will alter the landscape drastically in the side canyon. Debris from rock falls piles up at the bottom of the side canyons and is then carried down to the Colorado River the next time there is a flash flood. Rock falls frequently take out sections of trail in the Grand Canyon requiring the Park Service to close these trails until they can be repaired.(D)
    Once the ice has pushed the rocks off the edge and the water in the flash floods has carried them down to the river, then the Colorado itself takes over. The erosive action of the Colorado has been severely constrained by the building of the Glen Canyon Dam, which ended the annual spring floods, but there is still a lot of water flowing relatively quickly through a very narrow gorge. Before building the dam the Colorado River had spring floods that would exceed a flow rate of 100 000 CFS(cubic feet per second). All of that snow melting in the Colorado Rockies came pouring down through the Grand Canyon in May and June every year, like clock-work. These spring floods were considerably larger than today’s "trickle" of 8 000~10 000 CFS at low water and even the 20 000 CFS peak flow rates.
The author’s description of the Colorado River flow rate mentions which of the following?
选项 A、100 000 CFS before the building of the Dam.
B、8 000~20 000 CFS before the building of the Dam.
C、8 000~ 100 000 CFS after the building of the Dam.
D、20 000 CFS at peak rate after the building of the Dam.
答案D
解析 本题是事实信息题。原文关于科罗拉多河的流量有明确的说明，需要注意大坝建造前春季洪水的流速一般会超过100 000立方英尺／秒，并非刚好100 000立方英尺／秒，需排除A选项，选项C和选项B数据有误，因此选择D。
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