托福阅读真题+题目+答案解析：Stream Deposit

托福阅读真题+题目+答案解析：Stream Deposit

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 sedimentaccording 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.

Question 1 of 14

The word "curious " in the passage is closest in meaning to

A. strange

B. rapid

C. seasonal

D. essential  

你的答案：

正确答案：A

题目解析：

本题为词汇题。“curious”的意思是“好奇的；奇妙的”。词汇所在句“This downstream sorting is curious because stream velocity generally increases in the downstream direction.”的意思是“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。”下面我们来看选项： A选项“strange”意思是“奇怪的；奇妙的”，意思最接近，故为正确答案。 B选项“rapid”意思是“快速的”，意思不符，故排除。 C选项“seasonal”意思是“季节性的”，意思不符，故排除。 D选项“essential”意思是“必要的，本质的”，意思不符，故排除。

Question 2 of 14

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

A. downstream sorting decreases

B. flooding wanes

C. the speed of the current increases

D. the size of particles increases

你的答案：

正确答案：C

题目解析：

本题为事实信息题。题目问：河流的携带能力随着什么的增长而增长？根据题干中的定位词“competence”，可以分别定位到2句话。一句是第一段第2句话“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。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。”这两句话，都能说明河流的携带能力与速度有着密切联系。所以，这道题的正确答案是C选项：随着河流速度的增加，携带能力会增加。 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。 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。

Question 3 of 14

According to paragraph 1, all of the following are true of stream sorting EXCEPT

A. Most of the particles in mountain streams pile up behind boulders and cobbles

B. When particles of different sizes settle in a place, the smaller ones sit atop the larger ones.

C. There are generally more large particles upstream than downstream in a river.

D. In some situations, downstream particles are created from rocks that eroded as they traveled downstream.

你的答案：

正确答案：A

题目解析：

本题为否定事实信息题。我们可以从选项中找到关键词，回到原文中进行定位用排除法解题。 A选项，根据关键词“boulders and cobbles”定位到第一段中的这一句“Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of fine silt and clay.”这句话的意思是“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。”但是A选项说大多数山溪中的颗粒都在“boulders and cobbles”后面堆积起来了，这与原文是矛盾的，故A选项为正确答案。 B选项，根据关键词“atop”和“large”定位到第一段第3句话“If current velocity continues to decrease—as a flood wanes, for example—finer particles settle out on top of the large ones.”小颗粒确实会推挤在大颗粒上，故B选项符合原文，排除。 C选项，根据关键词“downstream”定位到这句话“Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of fine silt and clay.”这句话说，山溪通常被巨砾和鹅卵石阻塞，而下游的三角洲主要是由细泥和黏土堆积成的。很明显，上游的巨砾要比下游的细颗粒大。故C选项符合原文，排除。 D选项定位到倒数第2句话“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.” 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。选项中的“eroded”对应原文中的“abrasion”，故D选项也符合原文，排除。

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