托福阅读常考话题及练习题推荐六：地下水（2）

1  托福阅读常考话题：地下水练习题

Paragraph 1: A large, swift stream or river can carry all sizes of particles, from clay to boulders. When the current slows down, its competence (how much it can carry) decreases and the stream deposits the largest particles in the streambed. If current velocity continues to decrease - as a flood wanes, for example - finer particles settle out on top of the large ones. Thus, a stream sorts its sediment according to size. A waning flood might deposit a layer of gravel, overlain by sand and finally topped by silt and clay. Streams also sort sediment in the downstream direction. Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of fine silt and clay. This downstream sorting is curious because stream velocity generally increases in the downstream direction. Competence increases with velocity, so a river should be able to transport larger particles than its tributaries carry. One explanation for downstream sorting is that abrasion wears away the boulders and cobbles to sand and silt as the sediment moves downstream over the years. Thus, only the fine sediment reaches the lower parts of most rivers.

1.The word "curious" in the passage is closest in meaning to

A. strange

B. rapid

C. seasonal

D. essential

2.According to paragraph 1, the competence of a stream increases as

A. downstream sorting decreases

B. flooding wanes

C. the speed of the current increases

D. the size of particles increases

3.According to paragraph 1, all of the following are true of stream sorting EXCEPT

A. Most of the particles in mountain streams pile up behind boulders and cobbles

B. When particles of different sizes settle in a place, the smaller ones sit atop the larger ones.

C. There are generally more large particles upstream than downstream in a river.

D. In some situations, downstream particles are created from rocks that eroded as they traveled downstream.

4.The word "accumulate" in the passage is closest in meaning to

A. begin

B. appear

C. build up

D. spread

Paragraph 2: A stream deposits its sediment in three environments: Alluvial fans and deltas form where stream gradient (angle of incline) suddenly decreases as a stream enters a flat plain, a lake, or the sea; floodplain deposits accumulate on a floodplain adjacent to the stream channel; and channel deposits form in the stream channel itself. Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks. They commonly form in one year and erode the next. Rivers used for commercial navigation must be recharged frequently because bars shift from year to year. Imagine a winding stream. The water on the outside of the curve moves faster than the water on the inside. The stream erodes its outside bank because the current's inertia drives it into the outside bank. At the same time, the slower water on the inside point of the bend deposits sediment, forming a point bar. A mid-channel bar is a sandy and gravelly deposit that forms in the middle of a stream channel.

5.According to paragraph 2, which of the following is true about bars in streams?

A. They start forming in the stream channel and then expand over the banks.

B. They seldom form in rivers that are used for commercial navigation.

C. They tend to grow longer each year.

D. They often last no more than a year.

6.Why does the author ask the reader to "Imagine a winding stream"?

A. To explain how the presence of bars changes the speed and direction of water flow in a stream

B. To explain why bars are more common than alluvial fans or other types of sediment deposits

C. To illustrate the particular difficulties that commercial navigation faces on many rivers

D. To help explain how point bars are formed

Paragraph 3: Most streams flow in a single channel. In contrast, a braided stream flows in many shallow, interconnecting channels. A braided stream forms where more sediment is supplied to a stream than it can carry. The stream dumps the excess sediment, forming mid-channel bars. The bars gradually fill a channel, forcing the stream to overflow its banks and erode new channels. As a result, a braided stream flows simultaneously in several channels and shifts back and forth across its floodplain. Braided streams are common in both deserts and glacial environments because both produce abundant sediment. A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion. Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice. If a steep mountain stream flows onto a flat plain, its gradient and velocity decrease sharply. As a result, it deposits most of its sediment in a fan-shaped mound called an alluvial fan. Alluvial fans are common in many arid and semiarid mountainous regions.

7.According to paragraph 3, all of the following are true of braided streams EXCEPT

A. They shift back and forth on the floodplain as some of their channels fill with sediment and new ones are formed

B. They carry sediments that tend to be very similar in size.

C. They sometimes have mid-channel bars.

D. Some of their channels are created as a result of the overflow of other channels.

8.Why does the author include the information that "Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice"?

A. To give a reason why heavily sedimented braided streams are common in glacial environment

B. To explain why some mountain streams deposit most of their sediment in a fan-shaped mound

C. To identify the most common source of sediment in arid and semiarid mountainous regions

D. To help explain why glacial sediment decreases the gradient and velocity of steep mountain streams

9.The word "simultaneously" in the passage is closest in meaning to

A. in many directions

B. on the surface

C. at the same time

D. in a straight line