Official 52 Passage 1

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Stream Deposits

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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.

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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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  • 译文
  • 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。

    一条河流会在三种环境下发生沉淀:在河流进入平原、湖泊或大海的时候,河流坡度(倾斜的角度)会突然下降,在那些地方会形成冲积扇和三角洲;河漫滩沉积物会堆积在河道附近的漫滩上;河流内部也会形成河道沉积物。砂坝,即细长条的泥沙堆积物,是河流内部和河流两岸所形成的沉淀物的瞬态特征。砂坝通常在一年内形成,下一年就会被消磨掉。用于通商航行的河道必须频繁地修整,因为砂坝每年都会变。想象一条蜿蜒的河流。河流外缘的水流速度要比河流中心的水流速度快。河流会对其外围的河岸造成侵蚀,因为水流的惯性驱使河流冲向河岸。与此同时,河流内部较缓的水流会沉淀砂石,形成一个点坝。河心沙洲是在河道中央形成的,由沙子和碎石组成的沉积物。

    大多数的河流为单河道。相反,辫状河由很多浅的、互相连接的河道组成。当河流携带更多的泥沙,泥沙量超出河流的携带能力的时候,这些地方就会形成辫状河。河流会卸下多余的泥沙,形成河心沙洲。沙洲会渐渐填满河道,迫使水流溢出堤岸,侵蚀出新的河道。其结果是,辫状河在多个河道同时流动,并在河漫滩上来回流动。辫状河在沙漠和冰川环境中都很常见,因为沙漠和冰川都能产生大量的沉淀物。沙漠能产生大量沉积物,因为它几乎没有或者根本没有植被来防止其被侵蚀。冰川将基岩研磨成细小的沉积物,这些沉淀物会被冰川融化后形成的水流所携带。如果一条陡峭的山溪流到平坦的平原上,它的坡度和速度就会急剧下降。结果,它会把大部分的泥沙沉淀下来,形成一个扇形土丘,被称为冲积扇。冲积扇在许多干旱、半干旱的山区很常见。

    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。
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    解析

    段落大意:

    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)

    第三段:介绍网状河道和冲积扇的各自形成及常见区域

    第四段:介绍三角洲的形成(举例密西西比河)

     

    答案:ADF

    题型:小结题

    解析:

    选项A:对应到第一段后半部分内容;

    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C:对应第二段倒数第二句,但两者间的相互转化信息未提及;

    选项D:对应第三段内容;

    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;

    选项F:对应第四段内容。

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