Blocking Water

Is there a dam located near you?

1 If you are hiking through the woods and notice a pile of timber, mud, and stone blocking a stream, you just might have stumbled upon a beaver dam. Beavers build dams so they can construct their lodges in the middle of the pond that forms as a result of the blocked water. The entrance to the lodge is located under the surface of the water, allowing the pond to function as a natural defense against predators. The pond also allows the beavers to live right on top of water lilies, their favorite meal.

2 However, the biggest builders of dams are humans, and they have been doing it for a long time. Some of the earliest structures built by humans are dams dating back nearly 5,000 years. At the beginning of civilization, dams were built in the Middle East so that people could stay in one place and farm. The water captured by dams enabled the first farmers to irrigate crops in areas that were otherwise too dry. The Ma’rib Dam in Yemen, originally built 2,700 years ago, is still in use today, although many parts of it have been rebuilt during that time.

3 Irrigation is just one reason people build dams. Water that pools behind these structures can also supply drinking water to cities and towns. Dams allow large populations to live in places where water is scarce. Lake Mead is a reservoir, or artificial lake, created by Hoover Dam. It provides water for more than 16 million people in Nevada, Arizona, and California. The Colorado River feeds Lake Mead, which holds enough water to cover the entire state of Pennsylvania!

4 Dams are also used to help ships and barges navigate rivers that would otherwise be too shallow. Canals are built alongside the dam so that boats can travel around it. In addition, the canals usually contain locks because the area around the dam is higher than the river, meaning the boat needs to travel up and over the surrounding area.

5 A lock is part of the canal that can be blocked off and filled with water. Each lock is flooded or drained, depending on whether the boat needs to be raised or lowered. The front of the lock is then opened, allowing the boat to float into the next lock. The process is repeated until the boat has traveled up or down the area needed to pass the dam.

6 Many modern dams harness the power of flowing water, transforming its energy into electricity. These dams are hydroelectric dams. The largest hydroelectric dam in the world is under construction right now in China. It is expected to have a reservoir 370 miles long. The Three Gorges Dam on the Yangtze River is designed to control flooding but will also produce enough energy to fill a good deal of China’s electrical needs.

7 Three Gorges Dam has created controversy, for many of the same reasons that other large dams have. Land that has always been dry will now be underwater, altering the environment drastically. Dams especially affect migratory fish, like salmon and trout, because they need to move upstream and downstream for feeding and reproduction. Three Gorges Dam has also displaced millions of people because their homes have been destroyed to make space for the dam.

Spectrum Reading Grade 6

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1. What is one benefit to beavers of having the entrance to a lodge located underwater?

2. How old is the Ma’rib Dam?

3. Why do dams cause problems for migrating fish?

4. Why do canals often contain locks?

5. What are two reasons the Three Gorges Dam is being constructed in China?

6. What purpose would a reader have for reading this selection?

for pleasure or entertainment

for information

to learn how to build a dam

7. On the lines below, write a summary for paragraph 2.

8. Name one benefit and one drawback of the construction of Three Gorges Dam.

Spectrum Reading Grade 6

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