托福閱讀真題Official 52 Passage 1（六）

托福閱讀真題Official 52 Passage 1（六）

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 11 of 14

According to paragraph 4,what is true about river deltas?

A.They alternate between periods of spreading out and periods of getting smaller.

B.They have both active channels and channels that have been blocked by sediment.

C.They are typically formed by rivers that are braided before they reach the delta area.

D.They are much larger when formed in a still lake than when formed in the ocean.

正確答案：B

題目詳解

題型分類：事實信息題

原文定位：根據(jù)river deltas定位到第4段全段，可直接通過選項定位。

選項分析：

B選項對應(yīng)原文第6句，active channels對應(yīng)new channels develop，blocked by sediment對應(yīng)Sediment fills channels。

A選項的spreading out和smaller在第3、4句出現(xiàn)過，但是并未提及alternate，無中生有。

C選項before they reach the delta area與倒數(shù)第5句a stream feeding a delta or fan splits into many channels的時間順序矛盾。

D選項原文沒有提到much larger，無中生有。

Question 12 of 14

According to paragraph 4,what are engineers trying to accomplish in the Mississippi delta?

A.To expand the channels into which the river flows

B.To keep the river flowing in the existing channels

C.To control the amount of sediment the river brings to the delta

D.To increase the part of the delta that lies above water level

正確答案：B

題目詳解

題型分類：事實信息題

原文定位：根據(jù)engineers定位到最后一句。

選項分析：

最后一句說：工程師們建造了大量的堤壩，試圖加固河道。利用but一詞，結(jié)合上一句可知：是為了保持現(xiàn)有河道通暢，不再形成新的三角洲，即B選項。

A選項，定位區(qū)域沒有提到expand the channels，無中生有。

C選項，定位區(qū)域沒有提到control the amount of sediment，無中生有。

D選項，定位區(qū)域沒有提到increase the part of the delta，無中生有。

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