A large,swiftstreamor river can carry all sizes of particles, fromclayto boulders. When thecurrentslows down, itscompetence(how much it can carry) decreases and thestreamdeposits the largest particles in the streambed. Ifcurrentvelocitycontinues todecrease- as a flood wanes, for example - finer particlessettleout on top of the large ones. Thus, astreamsorts itssedimentaccording to size. A waning flood mightdepositalayerofgravel, overlain by sand and finally topped bysiltandclay. Streams alsosortsedimentin the downstream direction. Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of finesiltandclay. This downstream sorting iscuriousbecausestreamvelocitygenerally increases in the downstream direction. Competenceincreases withvelocity, so a river should be able totransportlarger particles than its tributaries carry. One explanation for downstream sorting is thatabrasionwears away the boulders and cobbles to sand andsiltas thesedimentmoves downstream over the years. Thus, only the finesedimentreaches thelowerparts of most rivers.

Astreamdeposits itssedimentin three environments:Alluvialfans and deltas form wherestreamgradient(angleof incline) suddenly decreases as astreamenters a flatplain, a lake, or the sea;floodplaindepositsaccumulateon afloodplainadjacentto thestreamchannel; andchanneldeposits form in thestreamchannelitself. Bars, which areelongatedmounds ofsediment, aretransientfeatures that form in thestreamchanneland on the banks. They commonly form in one year anderodethe next. Rivers used forcommercialnavigationmust be recharged frequently because barsshiftfrom year to year. Imagine a windingstream. The water on the outside of thecurvemoves faster than the water on the inside. Thestreamerodes its outside bank because thecurrent's inertia drives it into the outside bank. At the same time, the slower water on the inside point of the bend depositssediment, forming a point bar. A mid-channel bar is a sandy and gravellydepositthat forms in the middle of astreamchannel.

Most streams flow in a singlechannel. Incontrast, a braidedstreamflows in manyshallow, interconnecting channels. A braidedstreamforms where moresedimentis supplied to astreamthan it can carry. Thestreamdumps theexcesssediment, forming mid-channel bars. The barsgraduallyfill achannel, forcing thestreamto overflow its banks anderodenew channels. As a result, a braidedstreamflows simultaneously in several channels and shifts back and forth across itsfloodplain. Braided streams arecommonin both deserts andglacialenvironments because both produceabundantsediment. Adesertyields large amounts ofsedimentbecause it has little or novegetationto preventerosion. Glaciersgrindbedrockinto finesediment, which is carried by streams flowing from themeltingice. If asteepmountainstreamflows onto a flatplain, itsgradientandvelocitydecreasesharply. As a result, it deposits most of itssedimentin a fan-shapedmoundcalled analluvialfan. Alluvialfans arecommonin manyaridandsemiaridmountainousregions.

Astreamalso slowsabruptlywhere it enters the still water of a lake or ocean. Thesedimentsettles out to form a nearly flat landform called adelta. Part of thedeltalies above waterlevel, and theremainderliesslightlybelow waterlevel. Deltas are commonly fan-shaped, resembling the Greek letter "delta" (∆). Both deltas andalluvialfans change rapidly. Sedimentfills channels (waterways), which are then abandoned while new channels develop as in a braidedstream. As a result, astreamfeeding adeltaor fan splits into many channels called distributaries. A largedeltamayspreadout in thismanneruntil it covers thousands ofsquarekilometers. Most fans, however, are much smaller, covering afractionof asquarekilometer to a fewsquarekilometers. The Mississippi River has flowedthroughseven differentdeltachannels 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.

Questions:
question_stem: The word "curious" in the passage is closest in meaning to
choices: tag: A
text: strange

tag: B
text: rapid

tag: C
text: seasonal

tag: D
text: essential
answer_key: A
explanation: 【题目翻译】文章中“curious”这个词的意思最接近于以下哪个选项？
A：奇怪的
B：快速
C：季节性
D：必要的

【判定题型】：题目要求选出与高亮单词意思最接近的选项，故判断本题为词汇题。

【关键词定位】：根据关键词“curious”,定位到原句词汇所在句“This downstream sorting is curious because stream velocity generally increases in the downstream direction.”

【词汇解析】：词汇“curious”的意思是“好奇的”,带入到原句中，原句的意思是“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。”

【选项分析】
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_stem: According to paragraph 1, the competence of a stream increases as
choices: tag: A
text: downstream sorting decreases

tag: B
text: flooding wanes

tag: C
text: the speed of the current increases

tag: D
text: the size of particles increases
answer_key: C
explanation: 【题目翻译】根据第1段，流水的能力增加是因为什么？
A：下游分选减少
B：洪水减退
C：河流速度增加
D：颗粒的尺寸增大

【判定题型】：题目问的是文章中的具体细节信息，故根据题目问法可以判断本题为事实信息题。

【关键词定位】：根据关键词“competence”,定位到 Passage1。其中一句是原句为“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。
还有一句是第一段第2句话，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。”

【逻辑分析】这两句话，都能说明河流的携带能力与速度有着密切联系。随着河流速度的增加，携带能力会增加。

【选项分析】
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。 
C 选项：当河流速度变大时，它的携带能力也增强了，携带物就多了。C 正确。
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_stem: According to paragraph 1, all of the following are true of stream sorting EXCEPT
choices: tag: A
text: Most of the particles in mountain streams pile up behind boulders and cobbles

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

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

tag: D
text: In some situations, downstream particles are created from rocks that eroded as they traveled downstream.
answer_key: A
explanation: 【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。

question_stem: The word "accumulate" in the passage is closest in meaning to
choices: tag: A
text: begin

tag: B
text: appear

tag: C
text: build up

tag: D
text: spread
answer_key: C
explanation: 【题目翻译】文章中“累积”一词的意思最接近于以下哪个选项？
A：开始
B：出现
C：建立
D：传播

【判定题型】：题目要求选出与高亮单词意思最接近的选项，故判断本题为词汇题。

【关键词定位】：根据关键词“accumulate”,定位到原句 floodplain deposits accumulate on a floodplain adjacent to the stream channel.

【词汇解析】：词汇“accumulate”的意思是“积累”,带入到原句中，原句的意思是“河漫滩沉积物会堆积在河道附近的漫滩上。”

【选项分析】：
A 选项“begin”意思是“开始”，意思不符，故排除。 
B 选项“appear”意思是“出现”，意思不符，故排除。 
C 选项“build up”意思是“建立”，意思最接近，故为正确答案。 
D 选项“spread”意思是“传播”, 意思不符，故排除。

question_stem: According to paragraph 2, which of the following is true about bars in streams?
choices: tag: A
text: They start forming in the stream channel and then expand over the banks.

tag: B
text: They seldom form in rivers that are used for commercial navigation.

tag: C
text: They tend to grow longer each year.

tag: D
text: They often last no more than a year.
answer_key: D
explanation: 【题目翻译】根据2段，关于水流中的砂坝下列哪个选项是正确的？
A：它们开始在河道和在银行扩大成形。
B：它们很少形成河流，用于商业导航。
C：它们每年继续生长。
D：它们通常不持续超过一年。

【判定题型】：题目问的是文章中的具体细节信息，故根据题目问法可以判断本题为事实信息题。

【关键词定位】：根据关键词“ bars in streams”,定位到 Passage 2第二句，原句为“Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks.”意思是沙坝是细长的沙丘，是河道和河岸上形成的瞬态特征。

【逻辑分析】沙坝是瞬态特征，所以它们不会一直存在。文章中还说到砂坝通常在一年内形成，下一年就会被消磨掉。用于通商航行的河道必须频繁地修整，因为砂坝每年都会变。

【选项分析】
A 选项：砂坝一开始在河道中形成，然后延伸到岸上。错误，因为文中说“Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks.” 砂坝是河流内部和河流两岸所形成的沉淀物的瞬态特征。所以，在河流内和岸上，都可以形成砂坝，这两者是并列关系，不分先后。故 A 选项与原文不符，排除。 
B 选项：砂坝很少在用于通上航行的河道中形成。错误，因为文中说“Rivers used for commercial navigation must be recharted frequently because bars shift from year to year.” 用于通商航行的河道必须频繁地修整，因为砂坝每年都会变。这说明通上航行的河道中每年都会形成砂坝，故 B 选项与原文矛盾，排除。 
C 选项：它们每年都会增长。错误，因为文中说“They commonly form in one year and erode the next.” 砂坝通常在一年内形成，下一年就会被消磨掉。它们的位置可能会变换，但是不会增长，因为第二年就被消磨掉了。故 C 选项与原文不符，排除。 
D 选项：它们通常存在不超过一年。正确，出处同 C 选项“They commonly form in one year and erode the next.”这句话恰恰能说明砂坝存在不会超过1年的时间，故 D 选项符合原文，为正确答案。

question_stem: Why does the author ask the reader to "Imagine a winding stream"?
choices: tag: A
text: To explain how the presence of bars changes the speed and direction of water flow in a stream

tag: B
text: To explain why bars are more common than alluvial fans or other types of sediment deposits

tag: C
text: To illustrate the particular difficulties that commercial navigation faces on many rivers

tag: D
text: To help explain how point bars are formed
answer_key: D
explanation: 【题目翻译】为什么作者要求读者“想象一条蜿蜒的小溪”？
A：为了解释水柱的存在如何改变水流的速度和方向。
B：解释为什么沙洲比冲积扇或其他类型的沉积物更常见。
C：说明商业航行在许多河流上面临的特殊困难。
D：帮助解释砂坝是如何形成的。

【判定题型】：根据题干问法“why …… ask”，题目询问“Imagine a winding stream”(文章引用内容)的修辞目的，可以判断本题为修辞目的题。

【关键词定位】：根据关键词“Imagine a winding stream”,定位到 Passage x 第 x 句，原句为“Imagine a winding stream. The water on the outside of the curve moves faster than……the slower water on the inside point of the bend deposits sediment, forming a point bar.”

【引用内容分析】河流外缘的水流速度要比河流中心的水流速度快，以及河流外缘的水流和内部的水流分别造成的影响。从中我们得知，河流内部的水流因为较为缓慢，所以会沉淀砂石，形成点坝。所以从这段话中，我们可以知道点坝是如何形成的。

【选项分析】
A 选项：解释砂坝如何改变水流的速度和水流的方向。错误，因为文章没有说砂坝可以改变水流的速度的方向。故 A 选项未提及，排除。 
B 选项：解释为什么砂坝要比冲积扇或者其他形式的沉积物更普遍。错误，因为同样，这个比较关系在文中并没有提及，故 B 选项排除。 
C 选项：解释说明在很多河流中通商航行所遇到的困难。错误，因为文中只说用于通商航行的河道必须频繁地修整，因为砂坝每年都会变。并没有提到通上航行会遇到困难。故 C 选项未提及，排除。
D 选项：正是解释说明了点坝是如何形成的，符合原意，正确。

question_stem: According to paragraph 3, all of the following are true of braided streams EXCEPT
choices: tag: A
text: They shift back and forth on the floodplain as some of their channels fill with sediment and new ones are formed

tag: B
text: They carry sediments that tend to be very similar in size.

tag: C
text: They sometimes have mid-channel bars.

tag: D
text: Some of their channels are created as a result of the overflow of other channels.
answer_key: B
explanation: 【题目翻译】根据第3段，对于辫状河，下列各项均属实，除哪一项外？
A：由于一些河道淤积泥沙，形成了新的河道，它们在河漫滩上来回移动。
B：它们携带的沉积物往往大小非常相似。
C：它们有时会在中间通道的酒吧。
D：它们的一些通道是由于其他通道溢出而创建的。

【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“back and forth”定位到第3段这2句话“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.”

【选项分析】
A 选项中，这两句话说，沙洲会渐渐填满河道，迫使水流溢出堤岸，侵蚀出新的河道。结果是，辫状河在多个河道同时流动，并在河漫滩上来回流动。所以 A 选项完全符合原文，故排除。 
B 选项说，辫状河所携带的泥沙的大小非常类似。这在文中并没有提及，故 B 选项为正确答案。 
C 选项中，根据关键词“mid-channel bars”定位到这句话“The stream dumps the excess sediment, forming mid-channel bars.”辫状河因为携带了过多的泥沙，所以会形成河心沙洲。故 C 选项符合原文，排除。 
D 选项中，根据关键词“overflow”，定位到这一句“The bars gradually fill a channel, forcing the stream to overflow its banks and erode new channels.”由于河道被堵塞，所以河流会溢出堤岸，形成新的河道，因此 D 选项符合原文，排除

question_stem: Why does the author include the information that "Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice"?
choices: tag: A
text: To give a reason why heavily sedimented braided streams are common in glacial environment

tag: B
text: To explain why some mountain streams deposit most of their sediment in a fan-shaped mound

tag: C
text: To identify the most common source of sediment in arid and semiarid mountainous regions

tag: D
text: To help explain why glacial sediment decreases the gradient and velocity of steep mountain streams
answer_key: A
explanation: 【题目翻译】为什么作者要提到“冰川把基岩磨成细小的沉积物，而细小的沉积物是由融冰流出的小溪携带的”的信息？
A：为了解释为什么重度沉积的辫状河在冰川环境中很常见。
B：解释为什么有些山溪的大部分沉积物沉积在扇形的土丘中。
C：确定干旱和半干旱山区最常见的泥沙来源
D：为了帮助解释为什么冰川沉积物降低了陡峭山溪的坡度和速度。

【判定题型】：根据题干问法“why …… include”，题目询问“the information”(文章引用内容)的修辞目的，可以判断本题为修辞目的题。

【关键词定位】：根据关键词“xx”,定位到 Passage 3，定位到2句话：前一句话“A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion.”
再往前看一句“Braided streams are common in both deserts and glacial environments because both produce abundant sediment.”

【引用内容分析】前一句话“A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion.”这句话是在解释为什么沙漠地带能产生大量沉积物。
再往前看一句“Braided streams are common in both deserts and glacial environments because both produce abundant sediment.”这句话是说辫状河在沙漠和冰川环境中都很常见，因为沙漠和冰川都能产生大量的沉淀物。至此，这段话的结构已经非常明显：即先说明沙漠和冰川都能产生大量沉积物，然后再各用一句话解释它们为什么能产生沉积物。

【选项分析】
A 选项：解释为什么含有大量泥沙的辫状河在冰川环境很常见，正确。 
B 选项：解释为什么一些山溪会将泥沙沉淀在冲积扇地区。错误，因为冲积扇和题干这句话的关系不大，是下文新展开的一个层次，故 B 选项排除。 
C 选项：解释干旱和半干旱山区最常见的沉淀物来源。错误，因为题干这句话的目的，不是为了解释干旱和半干旱山区沉淀物来源，而是为了说明冰川和前文所提到的辫状河之间的联系，解释为什么辫状河在冰川环境下很常见。故 C 选项错误。 
D 选项：解释为什么冰川沉淀物会降低陡峭的山溪的坡度和水流速度。错误，因为文中并未提到该信息，故排除。

question_stem: The word "simultaneously" in the passage is closest in meaning to
choices: tag: A
text: in many directions

tag: B
text: on the surface

tag: C
text: at the same time

tag: D
text: in a straight line
answer_key: C
explanation: 【题目翻译】文章中“simultaneously”一词的意思最接近于
A 在很多方面
B 在表面上
C 同时
D 直线

【判定题型】：题目要求选出与高亮单词意思最接近的选项，故判断本题为词汇题。

【关键词定位】：根据关键词“simultaneously”,定位到原句“As a result, a braided stream flows simultaneously in several channels and shifts back and forth across its floodplain.”

【词汇解析】：词汇“simultaneously”的意思是“同一时间”,带入到原句中，原句的意思是“其结果是，辫状河在多个河道同时流动，并在河漫滩上来回流动。”

【选项分析】
A 选项“in many directions”意思是“在很多方向”，意思不符，故排除。 
B 选项“on the surface”意思是“在表面”, 意思不符，故排除。
C 选项“at the same time”意思是“同时”，意思符合，故为正确答案。 
D 选项“in a straight line”意思是“成直线”，意思不符，故排除。

question_stem: The word "abruptly" in the passage is closest in meaning to
choices: tag: A
text: noticeably

tag: B
text: gradually

tag: C
text: suddenly

tag: D
text: slightly
answer_key: C
explanation: 【题目翻译】文章中“abruptly”一词的意思最接近于以下哪个选项？
A：明显地
B：逐渐
C：突然之间
D：稍微

【判定题型】：题目要求选出与高亮单词意思最接近的选项，故判断本题为词汇题。

【关键词定位】：根据关键词“abruptly”,定位到原句“A stream also slows abruptly where it enters the still water of a lake or ocean.”

【词汇解析】：词汇“abruptly”的意思是“突然地”,带入到原句中，原句的意思是“当河流汇入湖泊或海洋中的静水时，水流速度也会骤然减慢。”

【选项分析】
A 选项“noticeably”意思是“显著地，明显地”, 意思不符，故排除。 
B 选项“gradually”意思是“逐步地，渐渐地”, 意思不符，故排除。 
C 选项“suddenly”意思是“突然地”，意思符合，故为正确答案。 
D 选项“slightly”意思是“轻微地”, 意思不符，故排除。

question_stem: According to paragraph 4, what is true about river deltas?
choices: tag: A
text: They alternate between periods of spreading out and periods of getting smaller

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

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

tag: D
text: They are much larger when formed in a still lake than when formed in the ocean.
answer_key: B
explanation: 【题目翻译】根据第4段，河流三角洲的真实情况是什么？
A 它们在扩张期和缩小期之间交替出现。

B 它们既有活跃的河道，也有被泥沙堵塞的河道。

C 它们通常是在到达三角洲地区之前由辫状河形成的。

D 它们在平静的湖里形成的时候比在海里形成的时候大得多。

【判定题型】：题目问的是文章中的具体细节信息，故根据题目问法可以判断本题为事实信息题。

【关键词定位】：根据关键词“river deltas”,定位到 Passage4，原句为“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.”意思是“一个大的三角洲可能以这种方式延伸，直到它覆盖了数千平方公里。然而，大多数风扇都小得多，覆盖了一小部分平方公里到几平方公里。”

【逻辑分析】这段话要表达的意思，其实是三角洲有大有小，但是没有说它会变大，然后又变小。那说明了河道不同才会导致三角洲的变化。

【选项分析】
A 选项：说的是时间，与原文不符，排除。 
B 选项：三角洲的河道有些很活跃，但也有些河道被泥沙堵住了。对应原文这一句“Sediment fills channels (waterways), which are then abandoned while new channels develop as in a braided stream.”这句话说了一些河道被堵住后，就像辫状河一样，新的河道会形成。故 B 选项符合原文，正确。 
C 选项：三角洲是由那些在到达三角地区之前就已经是辫状河的河流形成的。错误，因为很明显，文中说的是在形成三角洲之后，河流才会分流，形成支流，故 C 选项逻辑顺序颠倒，排除。 
D 选项：在静止湖泊中形成的三角洲要比在海洋中形成的三角洲更大。错误，因为文章完全没有提及这种比较关系。

question_stem: According to paragraph 4, what are engineers trying to accomplish in the Mississippi delta?
choices: tag: A
text: To expand the channels into which the river flows

tag: B
text: To keep the river flowing in the existing channels

tag: C
text: To control the amount of sediment the river brings to the delta

tag: D
text: To increase the part of the delta that lies above water level
answer_key: B
explanation: 【题目翻译】根据第4段，在密西西比三角洲，工程师们试图完成什么任务？
A：扩大河流入河道
B：保持河流在现有河道中流动
C：为了控制河流给三角洲带来的泥沙量
D：增加三角洲高于水位的部分。

【判定题型】：题目问的是文章中的具体细节信息，故根据题目问法可以判断本题为事实信息题。

【关键词定位】根据题干关键词“engineers”直接定位到文章最后一句话“But in recent years, engineers have built great systems of levees (retaining walls) in attempts to stabilize the channels.”这句话说近年来，工程师们建造了堤坝（挡土墙）系统以试图加固河道。

【逻辑分析】前文内容说，过去的5000到6000年内，密西西比河流经之处，已经形成了7个三角洲。而“But”表转折关系，所以工程师加固河道的目的显而易见，就是希望密西西比河不要再产生支流，形成新的三角洲。

【选项分析】
A 选项：扩张河道。错误，因为文中已经出现了“but”这个表示转折的逻辑词，再扩张河道，在逻辑上行不通。 
B 选项：保持河流在现有河道中流动，就是让密西西比河在已有的河道中流淌，不要产生新的支流，故 B 选项正确。 
C 选项：控制密西西比河带到三角洲的泥沙量。但文中并没有说加固河道和控制泥沙量有什么联系，故 C 选项未提及，排除。 
D 选项：增加三角洲在水下的部分。同理，该信息在文中未提及，排除。