小站备考
托福
托福阅读
Official52阅读真题

OFFICIAL52 According to paragraph 1, all of the following are true of stream sorting EXCEPT

展开
Stream Deposit
Tip:单击查看句义;划选/双击查生词
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.

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

你的答案:
正确答案:A
题目解析:
 后才能查看题目解析,还没有账号? 马上注册
【题目翻译】根据第1段,根据水流的储存能力,除了哪一项之外,以下所有内容都是正确的? A:山溪中的大部分颗粒堆积在巨石和鹅卵石后面。 B:当不同尺寸的粒子在一个地方沉降时,较小的粒子位于较大的粒子之上。 C:河流上游的大颗粒通常比下游多。 D:在某些情况下,下游的颗粒是由岩石在向下游移动时受到侵蚀而形成的。 【判定题型】:根据题干中出现的大写“EXCEPT”/"NOT" 等其他明显表示否定的词,可以判定本题为否定事实信息题,需要找出与原文信息不符的那个答案。 【关键词定位】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.” 【选项分析】: A选项,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。

学习页面

Medi

terr

anean

加强 + 政府 + 名词后缀

加强的政府——管理

原文例句

加入生词

本文生词 0

色块区域是你收藏过的生词;

查询次数越多,颜色越深哦~

显示文中生词

登录后才能收藏生词哦,现在登录注册>

本文重点词 45

文中加粗单词为本文重点词;

根据词频与核心词范围精心挑选,托福考试必掌握词汇。

显示文中重点词
学习本文词汇

文中划选/双击的生词、加粗重点词已收纳至词盒

可随时点击词盒查看哦~

只有在词句精学模式下才能开启词盒功能哦~

我知道了

词盒
收藏
笔记
我的笔记
5000
保存
反馈