A reservoir is a large lake created by people, usually by building a dam across a river valley. Cities build reservoirs to store water for drinking, farming, and other needs. When a dam blocks a river, water backs up behind it and gradually fills the valley, creating what looks like a natural lake but is actually carefully engineered.

Most cities need dependable water supplies even during droughts, when rivers might run low. A reservoir helps with this by storing water during rainy seasons so it's available year-round. The Hoover Dam, for example, created Lake Mead, an enormous reservoir that supplies water to millions of people in Nevada, Arizona, and California.

The word can also mean any supply or store of something kept for future use. A scientist might talk about a reservoir of knowledge in a particular field, meaning there's a deep store of information available. Your brain serves as a reservoir of memories and experiences you can draw on when needed. When you read widely and learn actively, you're building your own reservoir of ideas and understanding.