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OFFICIAL52 According to paragraph 4, what is true about river deltas?

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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.

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

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正确答案:B
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【题目翻译】根据第4段,河流三角洲的真实情况是什么? A它们在扩张期和缩小期之间交替出现。 B它们既有活跃的河道,也有被泥沙堵塞的河道。 C它们通常是在到达三角洲地区之前由辫状河形成的。 D它们在平静的湖里形成的时候比在海里形成的时候大得多。 【判定题型】:题目问的是文章中的具体细节信息,故根据题目问法可以判断本题为事实信息题。 【关键词定位】:根据关键词“river deltas”,定位到Passage4,原句为“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.”意思是“一个大的三角洲可能以这种方式延伸,直到它覆盖了数千平方公里。然而,大多数风扇都小得多,覆盖了一小部分平方公里到几平方公里。” 【逻辑分析】这段话要表达的意思,其实是三角洲有大有小,但是没有说它会变大,然后又变小。那说明了河道不同才会导致三角洲的变化。 【选项分析】 A选项:说的是时间,与原文不符,排除。 B选项:三角洲的河道有些很活跃,但也有些河道被泥沙堵住了。对应原文这一句“Sediment fills channels (waterways), which are then abandoned while new channels develop as in a braided stream.”这句话说了一些河道被堵住后,就像辫状河一样,新的河道会形成。故B选项符合原文,正确。 C选项:三角洲是由那些在到达三角地区之前就已经是辫状河的河流形成的。错误,因为很明显,文中说的是在形成三角洲之后,河流才会分流,形成支流,故C选项逻辑顺序颠倒,排除。 D选项:在静止湖泊中形成的三角洲要比在海洋中形成的三角洲更大。错误,因为文章完全没有提及这种比较关系。

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