1 . Cities usually have a good reason for being where they are, like a nearby port or river. People settle in these places because they are easy to get to and naturally suited to communications and trade. New York City, for example, is near a large harbour at the mouth of the Hudson River. Over 300 years its population grew gradually from 800 people to 8 million. But not all cities develop slowly over a long period of time. Boom towns grow from nothing almost overnight. In 1896, Dawson, Canada, was unmapped wilderness(荒野). But gold was discovered there in 1897, and two years later, it was one of the largest cities in the West, with a population of 30,000.

Dawson did not have any of the natural conveniences of cities like London or Paris. People went there for gold. They travelled over snow-covered mountains and sailed hundreds of miles up icy rivers. The path to Dawson was covered with thirty feet of wet snow that could fall without warming. An avalanche(雪崩) once closed the path, killing 63 people. For many who made it to Dawson, however, the rewards were worth the difficult trip. Of the first 20,000 people who dug for gold, 4,000 got rich. About 100 of these stayed rich men for the rest of their lives.

But no matter how rich they were, Dawson was never comfortable. Necessities like food and wood were very expensive. But soon, the gold that Dawson depended on had all been found. The city was crowded with disappointed people with no interest in settling down, and when they heard there were new gold discoveries in Alaska, they left Dawson City as quickly as they had come. Today, people still come and go — to see where the Canadian gold rush happened. Tourism is now the chief industry of Dawson City — its present population is 762.

2 . A giant dam was built many years ago to control the Colorado River in the U.S. This dam was built to protect the land and houses around the river.

This huge dam is in the Black Canyon. It is possible to drive a car from one side of the river to the other on a road, which is on the top of the dam. This dam is so big that there is an elevator inside. The elevator goes down forty-four stories from the road to the bottom. There is enough concrete in this dam to build a highway from New York to San Francisco. Thousands of people worked on this dam for five years.

This huge dam was called Boulder Dam when it was finished in 1936.Later it was renamed Hoover Dam in honor of a president of the United States. Hoover Dam, one of the highest dams in the world, is situated between the states of Arizona and Nevada.