River valleys are low-lying areas formed by the erosion and deposition of sediment by rivers over time. They typically have fertile soil, making them ideal for agriculture. The shape of a river valley can vary, with some being wide and flat, while others are narrow and steep-sided. Many of the world's ancient civilizations, such as those in the Nile River Valley and the Indus River Valley, developed in these regions due to the availability of water and resources. River valleys often support diverse ecosystems and provide habitats for various plants and animals, contributing to their ecological importance.