『新东方在线托福阅读』为托福考生准备了ETS托福TPO/Official 52 Passage 1阅读练习：According to paragraph 4, what are engineers trying to accomplish in the Mississippi delta?真题题目及答案解析，TPO/Official 52 Passage 1阅读原文范文音频翻译

托福TPO/Official 52 Passage 1阅读原文According to paragraph 4, what are engineers trying to accomplish in the Mississippi delta?题目答案及解析-新东方托福官方网校手机版

一条大而湍急的小溪或河流可以承载各种大小的颗粒，从微小的粘土到巨石。

A large, swift stream or river can carry all sizes of particles, from clay to boulders.

当水流速度减慢时，其能力（可承载的颗粒量）会降低，并且溪流会将最大的颗粒沉积在河床中。

When the current slows down, its competence (how much it can carry) decreases and the stream deposits the largest particles in the streambed.

如果水流速度继续降低（例如，洪水消退），则细颗粒会沉降在大颗粒的上面。

If current velocity continues to decrease—as a flood wanes, for example—finer particles settle out on top of the large ones.

因此，溪流根据大小将其沉淀物分类。

Thus, a stream sorts its sediment according to size.

逐渐减弱的洪水可能会沉积一层砾石，上面覆盖着沙子，最后是淤泥和粘土。

A waning flood might deposit a layer of gravel, overlain by sand and finally topped by silt and clay.

溪流还沿下游方向分流沉积物。

Streams also sort sediment in the downstream direction.

许多山间溪流被巨石和鹅卵石阻塞，但在下游，它们的三角洲主要由细粉砂和粘土组成。

Many mountain streams are choked with boulders and cobbles, but far downstream, their deltas are composed mainly of fine silt and clay.

这种下游分类很不同寻常的，因为水流速度通常在下游方向上增加。

This downstream sorting is curious because stream velocity generally increases in the downstream direction.

水流的承载能力随速度而增加，因此一条河流应该能够输送比其支流携带的更大的颗粒。

Competence increases with velocity, so a river should be able to transport larger particles than its tributaries carry.

对于下游分选的一种解释是，随着这些年来沉积物向下游移动，磨损会消磨巨石，使卵石变成沙和淤泥。

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.

因此，只有细小的泥沙才到达大多数河流的下游。

Thus, only the fine sediment reaches the lower parts of most rivers.

溪流在三种环境中沉积沉积物：冲积扇和三角洲形式，当溪流进入平坦的平原，湖泊或海洋时，溪流梯度（倾斜角度）突然减小；泛洪区沉积物堆积在与河道相邻的泛洪区上； 流通道本身会形成通道沉积物。

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.

沙洲是由沉积物堆积而成的细长土丘，是在河道和河岸上形成的瞬态特征。

Bars, which are elongated mounds of sediment, are transient features that form in the stream channel and on the banks.

它们通常在一年内形成，第二年就会被侵蚀。

They commonly form in one year and erode the next.

用于商业航行的河流必须经常重新勾画，因为条状沉积物每年都会变化。

Rivers used for commercial navigation must be recharted frequently because bars shift from year to year.

河流外侧的水比内侧的水移动得快。

The water on the outside of the curve moves faster than the water on the inside.

由于河流的惯性将河水驱赶到外侧，因此溪流侵蚀了外侧。

The stream erodes its outside bank because the current’s inertia drives it into the outside bank.

同时，溪流转弯处内侧点上较慢的水会沉积沉淀物，从而形成点坝。

At the same time, the slower water on the inside point of the bend deposits sediment, forming a point bar.

心滩坝是在河道中间形成的沙砾沉积物。

A mid-channel bar is a sandy and gravelly deposit that forms in the middle of a stream channel.

大多数河流在单个通道中流动。

相反，辫状流在许多浅的，相互连接的通道中流动。

In contrast, a braided stream flows in many shallow, interconnecting channels.

形成辫状流的地方，溪流携带的沉积物多于其携带的量。

A braided stream forms where more sediment is supplied to a stream than it can carry.

溪流将多余的沉积物倾泻而出，形成心滩坝。

The stream dumps the excess sediment, forming mid-channel bars.

这些条状沙坝逐渐填充河道，迫使流溢出其河岸并侵蚀新的通道。

The bars gradually fill a channel, forcing the stream to overflow its banks and erode new channels.

结果，辫状流在多个通道中同时流动，并在其洪泛区中来回移动。

As a result, a braided stream flows simultaneously in several channels and shifts back and forth across its floodplain.

辫状流在沙漠和冰川环境中都很常见，因为两者都会产生大量沉积物。

Braided streams are common in both deserts and glacial environments because both produce abundant sediment.

沙漠产生的沉积物很多，因为它很少或没有植被来防止侵蚀。

A desert yields large amounts of sediment because it has little or no vegetation to prevent erosion.

冰川将基岩磨成细小的沉积物，这些沉积物由融化的冰水流携带。

Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice.

如果陡峭的山流流入平坦的平原，其坡度和速度会急剧下降。

If a steep mountain stream flows onto a flat plain, its gradient and velocity decrease sharply.

结果，它将大部分沉积物沉积在一个称为冲积扇的扇形丘中。

As a result, it deposits most of its sediment in a fan-shaped mound called an alluvial fan.

冲积扇在许多干旱和半干旱山区很常见。

Alluvial fans are common in many arid and semiarid mountainous regions.

溪流进入湖泊或海洋的静水的地方也会突然减速。

A stream also slows abruptly where it enters the still water of a lake or ocean.

沉积物沉降形成一个几乎平坦的地形，称为三角洲。

The sediment settles out to form a nearly flat landform called a delta.

三角洲的一部分位于水位以上，其余部分位于水位以下。

Part of the delta lies above water level, and the remainder lies slightly below water level.

三角洲通常呈扇形，类似于希腊字母“三角洲”（Δ）。

Deltas are commonly fan-shaped, resembling the Greek letter “delta” (Δ).

三角洲和冲积扇的变化都很快。

Both deltas and alluvial fans change rapidly.

沉积物填充河道（水道），然后将其丢弃，而新河道如辫状流一样发展。

Sediment fills channels (waterways), which are then abandoned while new channels develop as in a braided stream.

结果是，一条形成三角洲或冲积扇的河流会分裂成多条河流，这些河流被称为支流。

As a result, a stream feeding a delta or fan splits into many channels called distributaries.

大三角洲可能会以这种方式散布开来，直到覆盖数千平方公里。

A large delta may spread out in this manner until it covers thousands of square kilometers.

但是，大多数冲积扇的体积要小得多，占地只有一平方公里到几平方公里。

Most fans, however, are much smaller, covering a fraction of a square kilometer to a few square kilometers.

在过去的5,000至6,000年中，密西西比河流经了七个不同的三角洲河道。

The Mississippi River has flowed through seven different delta channels during the past 5,000 to 6,000 years.

但是，近年来，工程师们建立了强大的堤防系统（挡土墙），试图稳固河道。

But in recent years, engineers have built great systems of levees (retaining walls) in attempts to stabilize the channels.

According to paragraph 1, the competence of a stream increases as

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

According to paragraph 2, which of the following is true about bars in streams?

According to paragraph 3, all of the following are true of braided streams EXCEPT:

Why does the author include the information that “Glaciers grind bedrock into fine sediment, which is carried by streams flowing from the melting ice”?

The word “simultaneously” in the passage is closest in meaning to

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

According to paragraph 4, what are engineers trying to accomplish in the Mississippi delta?

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.

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.

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” (Δ). 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.