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OFFICIAL52 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. Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it. To review the passage, click VIEW TEXT. Rivers can carry all sizes of particles that they deposit in characteristic ways.

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Stream Deposit
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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 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.

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" (∆). 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.

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. Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it. To review the passage, click VIEW TEXT. Rivers can carry all sizes of particles that they deposit in characteristic ways.

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

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

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

D.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.

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

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

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【题目翻译】说明:下面是文章的简要概括的介绍句。通过选择三个答案来完成总结,这三个答案表达了文章中最重要的观点。有些句子不属于摘要,因为它们表达了文章中没有呈现的想法,或者是文章中的次要思想。 河流可以携带所有尺寸的颗粒,它们以独特的方式沉积。 A:颗粒趋向于上游最大,下游最小,可能是因为水流向下游冲刷较大颗粒中的细颗粒。 B:沙洲通常从河岸上的点沙洲变成河道中间的沙洲。 C:流经干旱和半干旱山区的河流在河弯处往往形成冲积扇。 D:由于下游水流速度的增加,形成了各种各样的沉积物,包括沙坝、冲积扇和三角洲。 E:在流速降低的地方可以找到沉积物,当有过多的沉积物时,水流会变成辫状物,分成几个通道。 F:三角洲,形成于河流进入湖泊或海洋的地方,是自然不稳定的地形,可以延伸到大片地区。 【判定题型】:根据问题的提问方式和6选3的作答方式可以确定该题目为概要小结题。 【选项定位及分析】 A选项:沉积物颗粒在上游最大,在下游最小,这可能是因为自上而下的水流将大颗粒研磨成了精细的泥沙。对应文章第一段,文中第一段提到“Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of fine silt and clay.”,以及“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.”这两句话,所以A选项是第一段的概括总结,正确。 B选项:砂坝通常可以从河岸上的点坝转化为河心砂坝。“point bar”和“mid-channel bar”的概念在第二段段末提出,但是并没有提到两者可以互相转换。此外该信息属于细节性信息,可以直接排除。 C选项:干旱和半干旱山区会在河湾处形成冲积扇。对应第三段最后一句话“Alluvial fans are common in many arid and semiarid mountainous regions.”但是这是细节性信息,而且冲积扇也并不是在河湾处形成,文章说的是“If a steep mountain stream flows onto a flat plain”,当一条陡峭的山溪流到平坦的平原上,会沉淀泥沙形成冲积扇。故C选项排除。 D选项:很多类型的沉积物,例如砂坝、冲积扇、三角洲,都是因为下游河流的流速增加而造成的。这些沉积物的形成通常都与河流流速减慢有关系,故D选项与事实矛盾,排除。 E选项:当河流速度减慢时会形成沉积,当河流携带过量的泥沙时,可能会形成辫状河,形成多条河道。对应第三段内容,第三段提到“A braided stream forms where more sediment is supplied to a stream than it can carry.”,所以E选项是第三段的概括总结,正确。 F选项当河流进入到湖泊或海洋时会形成三角洲,三角洲是不太稳定的地貌,可以延伸出很大的一片区域。对应最后一段。最后一段说“Both deltas and alluvial fans change rapidly.”这句话中“change rapidly”说明三角洲地貌并不稳定,会增长得很快,最后一段还说“A large delta may spread out in this manner until it covers thousands of square kilometers.”正好对应F选项中所说的,三角洲会延伸出很大一片区域,故F选项是最后一段的总结,正确。

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