托福阅读真题Official 52 Passage 1（四）

托福阅读真题Official 52 Passage 1（四）

Stream Deposits

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.

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 recharted 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.

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.

A stream also slows abruptly where it enters the still water of a lake or ocean.The sediment settles out to form a nearly flat landform called a delta.Part of the delta lies above water level,and the remainder lies slightly below water level.Deltas are commonly fan-shaped,resembling the Greek letter“delta”(Δ).Both deltas and alluvial fans change rapidly.Sediment fills channels(waterways),which are then abandoned while new channels develop as in a braided stream.As a result,a stream feeding a delta or fan splits into many channels called distributaries.A large delta may spread out in this manner until it covers thousands of square kilometers.Most fans,however,are much smaller,covering a fraction of a square kilometer to a few square kilometers.The Mississippi River has flowed through seven different delta channels during the past 5,000 to 6,000 years.But in recent years,engineers have built great systems of levees(retaining walls)in attempts to stabilize the channels.

Question 7 of 14

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.

正确答案：B

题目详解

题型分类：否定事实信息题

原文定位：整个段落都在讨论braided streams，可直接通过选项定位。

选项分析：

B选项，原文没有提到similar in size，无中生有。

A选项出自于第五、六句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。

C选项出自于第四句The stream dumps the excess sediment,forming mid-channel bars。

D选项出自于倒数第五句The bars gradually fill a channel,forcing the stream to overflow its banks and erode new channels。

Question 8 of 14

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 environments

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

正确答案：A

题目详解

题型分类：修辞目的题

题干分析：本题考查句子之间的关系，需结合上下文来理解。

选项分析：

这一句描述的与glaciers有关，下文讲的是alluvial fan的形成过程，与glaciers无关，故需要看这一句前面的句子。braided streams这句说辫状河在沙漠和冰川环境中都很常见，之后desert和glaciers这两句话分别解释原因，故选A。

B选项fan-shaped mound与题干无关。

C选项arid and semiarid mountainous regions与题干无关。

D选项decreases the gradient and velocity无中生有。

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