Rivers are one of our most important natural resources . Many of the world's great cities are located on rivers, and almost every country has at least one river running through it that plays an important part in the lives of its people. Since the beginning of history, people have used rivers for transportation. The longest one in the United States is the Mississippi. The lifeline of Egypt is the Nile. To the people of India, the Ganges is great, but it is also important for transportation; ships can travel along it for a thousand miles. Other great rivers are the Congo in Africa and the Mekong in southeastern Asia. The greatest of all for navigation , however, is the Amazon in Brazil . It is so wide and so deep that large ships can go about two thousand miles up it. Besides transportation, rivers give food to eat, water to drink, water for crops, and chances for fun. In order to increase the supply  of water for crops, engineers sometimes build a dam  across a river and let a lake form behind the dam. Then people can use the water not only to irrigate  their fields but also to make electricity for their homes and industries. However, large cities and industries that are located on rivers often cause problems. As the cities grow in size and industries increase in number, the water in the rivers becomes polluted with chemicals and other materials. People are learning the importance, however, of doing more to keep their rivers clean if they want to enjoy the benefits of these natural resources. When rivers are dammed, people use the water for   _  .
keeping the rivers clean making electricity washing clothes irrigation and the production of electricity