【正文】 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 midchannel bar is a sandy and gravelly deposit that forms in the middle of a stream channel. Most streams flow in a single channel. 4. The word “accumulate” in the passage is closest in meaning to ? Begin ? Appear ? Build up ? Spread 5. According to paragraph2, 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 mercial 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 mon than alluvial fans or other types of sediment deposits ? To illustrate the particular difficulties that mercial navigation faces on many rivers ? To help explain how point bars are formed Paragraph 3: 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 midchannel 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 mon 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 fanshaped mound called an alluvial fan. Alluvial fans are mon in many arid and semiarid mountainous regions. 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 midchannel 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 mon in glacial environments ? To explain why some mountain streams deposit most of their sediment in a fanshaped mound ? To identify the most mon 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 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 monly fanshaped, resembling the Greek letter delta (!〇 ). 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 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 ? 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 acplish 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. 14. Directions: An introductory sentence for a brief summary of the passage 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 answer choices 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 choices to the spaces wh