托福阅读常考话题及练习题推荐七：地下水（3）

1托福阅读常考话题：地下水练习题

Paragraph 4: 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.

10.The word "abruptly" in the passage is closest in meaning to

A. noticeably

B. gradually

C. suddenly

D. slightly

11.According to paragraph 4, what is true about river deltas?

A. They alternate between periods of spreading out and periods of getting smaller

B. They have both active channels and channels that have been blocked by sediment.

C. They are typically formed by rivers that are braided before they reach the delta area.

D. 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?

A. To expand the channels into which the river flows

B. To keep the river flowing in the existing channels

C. To control the amount of sediment the river brings to the delta

D. To increase the part of the delta that lies above water level

Paragraph 4: 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. [■]

13.Look at the four squares [▇] that indicate where the following sentence could be added to the passage. Where would the sentence best fit? Click on a square [▇] to add the sentence 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.

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.

Answer Choices

A. Particles tend to be largest upstream and smallest downstream, probably because water flowing downstream erodes fine sediment from the larger particles.

B. Bars commonly shift from being point bars on a river bank to being mid-channel bars

C. Rivers that flow through arid and semiarid mountainous regions tend to form alluvial fans at bends in the river.

D. 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.

E. Deposits of sediment are found where flow velocity decreases, and when there is excessive sediment a stream may become braided, dividing into several channels.

F. Deltas, which are formed where streams enter lakes or the ocean, are naturally unstable landforms that can extend over a large area.