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OFFICIAL52 Look at the four squares [▇] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [▇] to add the sentence to the passage.

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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. [■]

13.Look at the four squares [▇] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [▇] to add the sentence 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..

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【题目翻译】看四个正方形[],它们指示了下列句子可以加到文章中的什么地方。这个句子最适合哪儿? A:如果密西西比河不被这些系统所控制,它很可能会放弃它现在的路径,切入附近河流向西的通道 文章最后一段翻译: 当河流汇入湖泊或海洋中的静水时,水流速度也会骤然减慢。泥沙会沉淀下来,形成一个近乎平坦的地貌,被称为三角洲。三角洲的一部分在水面之上,而剩余的部分略微低于水面。三角洲通常为扇形,形状和希腊字母“∆”很类似。三角洲和冲积扇都变化得很快。沉积物会填满河道(水道),然后这些河道就会被废弃,而新的河道就会像辫状河那样形成。结果是,一条形成三角洲或冲积扇的河流会分裂成多条河流,这些河流被称为支流。一个大三角洲可能会以这种方式延展开,直到占地数千平方公里。然而,大多数冲积扇,规模要小得多,覆盖一平方公里至几平方公里的小片土地。过去的5000到6000年内,密西西比河流经之处,已经形成了7个三角洲。但是近年来,工程师们建造了堤坝(挡土墙)系统以试图加固河道。 【判定题型】根据题目问法,题目要求将句子插入到文中最恰当的空格处,故判断本题为句子插入题。 【待插入句分析】插入句的意思是“如果没有这种系统来控制密西西比河的话,密西西比河会流入附近向西方流去的河流中。”很明显,这句话是在假设如果没有堤坝系统的话会发生怎样的后果。 【原文分析】只有文章最后一句话提到了堤坝系统。待插入句中“such”一词是指示代词,肯定是在指代前文所出现过的内容。 【选项分析】A、B、C三个选项没有提到堤坝系统,只有D选项提到了,所以D选项正确。

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