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TPO52托福阅读Passage1原文文本
Stream Deposits
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" (∆). 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. {[3|13|A]} Most fans, however, are much smaller, covering a fraction of a square kilometer to a few square kilometers. {[3|13|B]} The Mississippi River has flowed through seven different delta channels during the past 5,000 to 6,000 years. {[3|13|C]} But in recent years, engineers have built great systems of levees (retaining walls) in attempts to stabilize the channels.{[3|13|D]}
TPO52托福阅读Passage1题目
1.The word "curious" in the passage is closest in meaning to
strange
rapid
seasonal
essential
2.According to paragraph 1, the competence of a stream increases as
downstream sorting decreases
flooding wanes
the speed of the current increases
the size of particles increases
3.According to paragraph 1, all of the following are true of stream sorting EXCEPT:
Most of the particles in mountain streams pile up behind boulders and cobbles.
When particles of different sizes settle in a place, the smaller ones sit atop the larger ones.
There are generally more large particles upstream than downstream in a river.
In some situations, downstream particles are created from rocks that eroded as they traveled downstream.
4.The word "accumulate" in the passage is closest in meaning to
begin
appear
build up
spread
5.According to paragraph 2, which of the following is true about bars in streams?
They start forming in the stream channel and then expand over the banks.
They seldom form in rivers that are used for commercial navigation.
They tend to grow longer each year.
They often last no more than a year.
6.Why does the author ask the reader to "Imagine a winding stream"?
To explain how the presence of bars changes the speed and direction of water flow in a stream
To explain why bars are more common than alluvial fans or other types of sediment deposits
To illustrate the particular difficulties that commercial navigation faces on many rivers
To help explain how point bars are formed
7.According to paragraph 3, all of the following are true of braided streams EXCEPT:
They shift back and forth on the floodplain as some of their channels fill with sediment and new ones are formed.
They carry sediments that tend to be very similar in size.
They sometimes have mid-channel bars.
Some of their channels are created as a result of the overflow of other channels.
8.Why does the author include the information that "Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice"?
To give a reason why heavily sedimented braided streams are common in glacial environments
To explain why some mountain streams deposit most of their sediment in a fan-shaped mound
To identify the most common source of sediment in arid and semiarid mountainous regions
To help explain why glacial sediment decreases the gradient and velocity of steep mountain streams
9.The word ""simultaneously"" in the passage is closest in meaning to
in many directions
on the surface
at the same time
in a straight line
10.The word "abruptly" in the passage is closest in meaning to
noticeably
gradually
suddenly
slightly
11. According to paragraph 4, what is true about river deltas?
They alternate between periods of spreading out and periods of getting smaller
They have both active channels and channels that have been blocked by sediment.
They are typically formed by rivers that are braided before they reach the delta area.
They are much larger when formed in a still lake than when formed in the ocean.
12.According to paragraph 4, what are engineers trying to accomplish in the Mississippi delta?
To expand the channels into which the river flows
To keep the river flowing in the existing channels
To control the amount of sediment the river brings to the delta
To increase the part of the delta that lies above water level
13.Look at the four squares [▇] that indicate where the following sentence could be added to the passage.
If the Mississippi River were not contained by such systems, it would probably abandon its present path and cut into the channel of a nearby river to the west.
Where would the sentence best fit? Click on a square [▇] to add the sentence to the passage.
14.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.
Drag your answer choices to the spaces where they belong. To remove an answer choice, click on it.
To review the passage, click VIEW TEXT.
Rivers can carry all sizes of particles that they deposit in characteristic ways.
Particles tend to be largest upstream and smallest downstream, probably because water flowing downstream erodes fine sediment from the larger particles.
Bars commonly shift from being point bars on a river bank to being mid-channel bars.
Rivers that flow through arid and semiarid mountainous regions tend to form alluvial fans at bends in the river.
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.
Deposits of sediment are found where flow velocity decreases, and when there is excessive sediment a stream may become braided, dividing into several channels.
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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