80. Since the beginning of history, people have used rivers for transportation. This is true not only of the great rivers, such as the Nile in Egypt and the Ganges in India, but also of the innumerable smaller rivers all around the world. Besides transportation, rivers provide food, water for crops, water to drink, and recreational opportunities for people who live along their banks. Rivers are destined to reach the sea, but dams have been built on many rivers to provide water for irrigation and electrical power for homes and industries.
The main topic of this paragraph is ________________.