托福阅读真题Official 52 Passage 1（七）

托福阅读真题Official 52 Passage 1（七）

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 13 of 14

Look at the four squares[■]that indicate where the following sentence could be added to the passage.If the Mississippi River were not contained by such systems,it would probably abandon its present path and cut into the channel of a nearby river to the west.Where would the sentence best fit?Click on a square[■]to add the sentence to the passage.

正确答案：D

题目详解

题型分类：插入句子题

题干分析：考察句子和句子之间的衔接判断（通过such systems可以预测出前面一定提到过systems）。

选项分析：

D选项systems of levees呼应待插入句such systems。

A,B,C选项逻辑错误，得先提到systems，再说such systems，故不能插入

Question 14 of 14

Directions:An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage.Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.This question is worth 2 points.

Rivers can carry all sizes of particles that they deposit in characteristic ways.

Answer Choices:

A.

Particles tend to be largest upstream and smallest downstream,probably because water flowing downstream erodes fine sediment from the larger particles.

B.

A wide variety of deposits,including bars,alluvial fans,and deltas,are formed as a result of an increase in the speed of the downstream current.

C.

Bars commonly shift from being point bars on a river bank to being mid-channel bars.

D.

Deposits of sediment are found where flow velocity decreases,and when there is excessive sediment a stream may become braided,dividing into several channels.

E.

Rivers that flow through arid and semiarid mountainous regions tend to form alluvial fans at bends in the river.

F.

Deltas,which are formed where streams enter lakes or the ocean,are naturally unstable landforms that can extend over a large area.

正确答案：ADF

题目详解

题型分类：文章总结题

题干分析：选择概括性的正确选项。

选项分析：

A选项对应第一段重要信息，是第一段第七句以后内容的总结。第7句many mountain streams...boulders and cobbles，but far downstream...silt and clay对应选项中的largest upstream，smallest downstream；倒数第二句的wears away the boulders and cobbles to sand and silt对应选项中的erodes fine sediment from the larger particles。

D选项对应第三段重要信息，第3句的A braided stream forms where more sediment is supplied to a stream than it can carry对应选项中的when there is excessive sediment a stream may become braided，倒数第2、3句的velocity decrease sharply...it deposits most of its sediment对应选项中的Deposits of sediment are found where flow velocity decreases。

F选项对应第四段重要信息，第5句的Both deltas and alluvial fans change rapidly对应选项中的naturally unstable landforms，倒数第4句的A large delta may spread out in this manner对应选项中的extend over a large area。

B选项出自第二段第一句，但原文说的是where stream gradient(angle of incline)suddenly decreases，而不是increase in the speed of the downstream current，与原文矛盾。

C选项出自第二段倒数第1、2句，但是选项中的shift无中生有。

E选项出自第三段最后一句话，为第三段细节。

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