The Nile is one of the longest rivers in the world. It flows north through northeastern Africa and ends in the Mediterranean Sea. For thousands of years, people have lived along its banks because the land beside the river is very fertile. Every year in ancient times, the river flooded and left rich soil on the fields. Farmers then planted wheat, barley and other crops. In a country with very little rain, the river was almost the only source of water, so towns and cities grew close to it. Today, a large dam in southern Egypt controls the water, so the yearly flood no longer happens in the same way. The dam also produces electricity and stores water for the dry months. However, some people point out that the soil now receives less natural fertilizer, and farmers must use more chemicals. Even so, the Nile remains the center of life for millions of people.

Question: What is the main idea of the passage?