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OFFICIAL52 According to paragraph 1, the competence of a stream increases as

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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.

2.According to paragraph 1, the competence of a stream increases as

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【题目翻译】根据第1段,流水的能力增加是因为什么? A:下游分选减少 B:洪水减退 C:河流速度增加 D:颗粒的尺寸增大 【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“competence”,定位到Passage1。其中一句是原句为“When the current slows down, its competence (how much it can carry) decreases……”,意思是当水流放缓的时候,其携带能力(能够携带的颗粒量)会减弱。 还有一句是第一段第2句话,第二句话是“Competence increases with velocity, so a river should be able to transport larger particles than its tributaries carry.”,意思是“水流的携带能力随着流速的加快而提升,因此一条河流可以输送的颗粒应该比它的支流所携带的颗粒更大。” 【逻辑分析】这两句话,都能说明河流的携带能力与速度有着密切联系。随着河流速度的增加,携带能力会增加。 【选项分析】 A选项:顺流而下沉淀物分类的减少。错误,因为文中只提到“This downstream sorting is curious because stream velocity generally increases in the downstream direction.”,说明水流的速度沿着下游方向越来越快。但是下游方向沉淀物分类的减少,与水流的携带能力无关。故A选项排除。 B选项:洪水消退。我们根据关键词“flood wanes”,定位第一段第3句“If current velocity continues to decrease—as a flood wanes, for example—finer particles settle out on top of the large ones.”这句话说,洪水消退时水流速度会减慢。但是洪水消退和水流的携带能力并无直接关系。其次,根据后文提到的“Competence increases with velocity”,我们可以判断,洪水消退时,水流速度减慢,所以其携带能力也会减弱,所以与题干矛盾。综上,B选项错误。 C选项:当河流速度变大时,它的携带能力也增强了,携带物就多了。C正确。 D选项:颗粒大小的增加。错误,因为本段中没有提到颗粒大小与水流携带能力之间的关系,属于未提及的内容。故D选项排除。

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