B

The United States is rich in water resources，and its many lakes and rivers have played an important role in the development of the nation’s economy.West of the northern Appalachians lie five Great Lakes：Lake Superior，Lake Michigan，Lake Huron，Lake Erie and Lake Ontario.The lakes are shared between the United States and Canada except Lake Michigan，which lies entirely within the United States.Together，they hold more than 20％ of the world’s fresh surface water.The lakes are connected to the Atlantic Ocean by the St.Lawrence River in Canada.In the United States，the Erie Canal connects Lake Erie to the Hudson River，which flows into the Atlantic Ocean at New York City.Before the early part of the 20th century，the canal was a very important water route for transporting agricultural products from the American Midwest to New York.

the Mississippi River

The Mississippi River is the largest river system in the United States，and is sometimes called the“Father of all Waters”.It flows about 3，800 kilometers from Minnesota to the Gulf of Mexico.Together with its longest tributary，the Missouri River，it is 5，970 kilometers long.The Mississippi River system drains almost the entire area between the Appalachians and the Rockies，more than half of the continental United States.The most important tributary in the west is the Missouri River，which has its source in the Rocky Mountains and enters the Mississippi at St.Louis.In the east，the Ohio River is the main tributary;the Ohio is also notable for having served as the dividing line between the North and the South in the era before the Civil War.

Other important rivers include the Rio Grande，the Colorado River and the Columbia River.The Rio Grande forms the border between Texas and Mexico.The Colorado River runs through the Grand Canyon National Park as mentioned above;it flows through some of the driest parts of the United States，and its water is therefore in great demand.The Columbia River originates in western Canada and continues in the United States for 2，000 kilometers.It has the greatest hydroelectric potential of any waterway in North America and has 11 dams constructed on it.