The Rio Grande is one of the most important rivers in the US. But unlike other     1     （significance）rivers, it is not used for shipping.       2     makes the Rio Grande so important is its location. Since 1848, it     3     （be）the official border of Texas, the US and Mexico.

The river,     4     （measure）around 1,900 miles, is the     5     （five）longest in North America. Because the river twists（蜿蜒）so much, it     6     （occasion）changes its course. The river starts in Colorado and extends downward to the Gulf of Mexico.

The Rio Grande today is mostly used     7     a source of drinking water. Sadly, much of the water has been used up and parts of the river are almost dry! This is because people take     8     （much）water from the river than the river can get back from rain and other sources. Experts are working to correct this, though, with hopes of restoring the river to its past     9     （strong）.

Today, the Rio Grande is a popular tourist destination. Though borders like the Rio Grande separate the US and Mexico, they are also shared spaces. The Rio Grande is therefore     10     symbol of friendship and peace between two peoples.

Yellow River Valley is the birthplace of ancient Chinese     1    （civilize）, and for that reason the Yellow River is often called "Mother River." The valley     2    （surround）the principal river of northern China and is at the center of thousands of years of Chinese history. It is called the Yellow River     3     its waters carry soil, which gives the river     4    （it）yellow-brown color, and when the river overflows, it leaves a yellow residue（残留）behind. While the river helps create fertile land that is suitable     5     farming, during certain times of the year the Yellow River frequently overflows. The water damages     6    （house）and crops across the North China Plain, an important     7    （agriculture）region. For thousands of years, the Chinese have carried out major public work projects     8    （control）the water from the Yellow River. Despite the possibility of flooding, many of China's cities,     9    （situate）along the river, are growing increasingly in the valley,     10     have become the center of Chinese culture, society, and learning.

5 . If there is a building which symbolizes a country—such as the Eiffel Tower for France and Sydney Opera House for Australia—then it has to be the Taj Mahal for India.

It was set up by Emperor Shah Jahan in 1653 in memory of his wife, Mumtaz Mahal. From the time they got married, they wouldn’t be separated. She followed him to wars, advised him on affairs of state, and was loved by his people for her good work. But she died in 1631 during childbirth. The emperor was heartbroken and had the Taj Mahal built as a sign of his love.

It took more than 20 years for the Taj Mahal to be built. Workers were brought in, not only from all over India, but from Central Asia too. A total of 20,000 people worked on the building.

In 1657, Shah Jahan fell ill, and in 1658 his son Aurangzeb, imprisoned (囚禁) his father and seized power. Shah Jahan stayed in prison until his death in 1666. He was finally buried there with his wife he could never forget.

The Taj Mahal is definitely worth more than a single visit. As it is built with white marble stones, its character changes in different lights. It looks more beautiful at sunrise and sunset. At sunset, for example, the color of the Taj changes from white to yellow, then to pink. As the moon rises, it turns a silvery white.

To show respect to the Taj, tourists are asked to take off their shoes during their visit.

9 . A dam is a man-made structure built across a river. Most dams are built to control a river’s water flow, improve navigation and control flooding. However, some dams are built to produce hydro-electric power.

Hydro-electric power is produced as water passes through a dam, and into a river below. The more water that passes through a dam, the more energy that is produced. Once a dam is built, a man-made lake is created behind the dam.

Electricity is produced by a kind of equipment called a turbine(涡轮机).Turbines contain metal coils(线圈)surrounded by magnets(磁铁). When the magnets move round rapidly over the metal coils, electricity is produced. Turbines are located inside dams. The falling water makes the magnets go around the coils.

Dams provide clean energy, but they can also harm the environment. Species that use rivers to reproduce are often hurt by dams. In the northwest of the US, the population of fishes has dropped from 16 million to 2.5 million since hydro-electric plants were built on the Columbia River. Dams all over the world have hurt some species.

The highest dam in the United States is located near Oroville, California. The Oroville Dam towers 230 meters and is more than a mile wide. This dam was built in 1968, 22 years after the Hoover Dam. The Hoover Dam, on the Nevada-Arizona border controls the Colorado River. It is 221 meters high and has 2.6 million hectare-meter(公顷)of water.

The highest dam in the world is the Nurek Dam on the Vakhsh River in Tajikistan, a country in central Asia. This dam is 300 meters tall.