Official 52 Passage 1


Stream Deposits


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.

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Rivers can carry all sizes of particles that they deposit in characteristic ways.

正确答案: A D F
  • A.
    Particles tend to be largest upstream and smallest downstream, probably because water flowing downstream erodes fine sediment from the larger particles.
  • B.
    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.
  • C.
    Bars commonly shift from being point bars on a river bank to being mid-channel bars.
  • D.
    Deposits of sediment are found where flow velocity decreases, and when there is excessive sediment a stream may become braided, dividing into several channels.
  • E.
    Rivers that flow through arid and semiarid mountainous regions tend to form alluvial fans at bends in the river.
  • 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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  • 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.

  • 一条大而湍急的溪流或河流可以携带各种大小的颗粒,小到粘土,大到巨石。当水流放缓的时候,其携带能力(能够携带的颗粒量)会减弱,最大的颗粒会沉淀在河床上。如果水流的速度继续减弱——比如洪水消退时——更细的颗粒会沉淀在大颗粒上。因此,溪流按大小将沉积物分类。洪水消退时最先可能会沉积一层砾石,接着沙子会覆盖在砾石之上,最后淤泥和黏土会覆盖在最上面。河流也会沿着下游的方向将沉淀物分类。许多山间的溪流会被巨砾和鹅卵石阻塞,但在更远处的下游,三角洲主要是由细泥和黏土堆积成的。这种顺流而下的分类是很奇妙的,因为河流的速度通常在下游方向加快。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。对这种顺流而下分类的一种解释是,随着沉淀物年复一年地向下游移动,水流的摩擦力将巨砾和卵石磨成了沙子和淤泥。因此,只有细泥沙能到达大多数河流的下游。



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    24 感谢 不懂


    第一段:介绍河流沉积及河流对沉积物分类的方式:1)根据size 2)在下游方向及其原理

    第二段:河流沉积的区域:1)冲积扇和三角洲2)河漫滩 3)河道; 河道沉淀障碍(bar)的形成介绍(举例:蜿蜒的溪流,外侧水流比内侧快)








    选项B:信息错误“are formed as a result of an increase in the speed of the downstream current”与第二段前两句信息不符;



    选项E:“Rivers that flow through arid and semiarid mountainous regions tend to form alluvial fans”对应第三段最后一句,但后面“at bends in the river.”是在第二段的内容,不是alluvial fans的信息,信息混淆;