【正文】 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 monly form in one year and erode the next. Rivers used for mercial 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 current39。 floodplain deposits accumulate on a floodplain adjacent to the stream channel。TPO371 The Emergence of Civilization Starting around 8000 ., the most extensive exploitation of agriculture occurred in river valleys, where there were both good soil and a dependable water supply regardless of the amount of rainfall. In the Near East, this happened in the Fertile Crescent, the region extending up the Nile Valley in Egypt, north through the Levant (Palestine, Lebanon, and Syria), and southeast into the Tigris and Euphrates river valleys of Mesopotamia. The richest soil was located in the deltas at the mouths of the rivers, but the deltas were swampy and subject to flooding. Before they could be farmed, they needed to be drained and irrigated, and floodcontrol systems had to be constructed. These activities required administrative anization and the ability to mobilize large pools of labor. In Mesopotamia, perhaps as a consequence of a period of drought, massive landuse projects were undertaken after 4000 . to cultivate the rich delta soils of the Tigris and Euphrates Rivers. The land was so productive that many more people could be fed, and a great population explosion resulted. Villages grew into cities of tens of thousands of persons. These large cities needed some form of centralized administration. Archaeological evidence indicates that the anization initially was provided by religion, for the largest building in each city was a massive temple honoring one of the Mesopotamian gods. In Uruk, for example, a 60foot long temple known as the White House was built before 3000 . There were no other large public buildings, suggesting that the priests who were in charge of the temples also were responsible for governing the city and anizing people to work in the fields and on irrigation projects building and maintaining systems of ditches and dams. The great concentration of wealth and resources in the river valleys brought with it further technological advances, such as wheeled vehicles, multicolored pottery and the pottery wheel, and the weaving of wool garments. Advances in metal technology just before 2020 . resulted in the creation of bronze, a durable alloy (or mixture) of about 90 percent copper and 10 percent tin that provided a sharp cutting edge for weapons. By 3000 ., the economies and administrations of Mesopotamia and Egypt had bee so plex that some form of record keeping was needed. As a result, writing was invented. Once a society became literate, it passed from the period known as prehistory into the historic period. In fact, the word history es from a Greek word meaning narrative people could not provide a detailed permanent account of their past until they were able to write. The totality of these developments resulted in the appearance, around 300 ., of a new form of culture called civilization. The first civilizations had several defining characteristics. They had economies based on agriculture. They had cities that functioned as administrative centers and usually had large populations. They had different social classes, such as free persons and slaves. They had specialization of labor, that is, different people serving, for example, as rulers, priests, craft workers, merchants, soldiers, and farmers. And they had metal technology and a system of writing. As of 3000 ., civilization in these terms existed in Mesopotamia, Egypt, India, and China. This first phase of civilization is called the Bronze Age because of the importance of metal technology. The most characteristic Near Eastern Bronze Age civilizations, those of Mesopotamia and Egypt, were located in river valleys, were based on the extensive exploitation of agriculture, and supported large populations. Bronze was a valuable modity in these civilizations。 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 monly form in one year and erode the next. Rivers used for mercial 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 current39。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