A watershed is an area of land where all the rain, snow, and melting ice drains downhill into the same body of water, like a river, lake, or ocean. Picture your neighborhood after a rainstorm: water runs down streets, into storm drains, and eventually flows to the same place. That's how a watershed works, but on a much larger scale.

The Mississippi River watershed, for example, covers nearly half of the United States. Every stream, creek, and river in that enormous area eventually feeds into the Mississippi. When it rains in Montana or Minnesota, that water might travel thousands of miles before reaching the Gulf of Mexico, but it's all part of the same watershed.

Understanding watersheds matters because what happens upstream affects everyone downstream. If a factory pollutes a stream in one state, that pollution travels through the watershed and can harm communities hundreds of miles away.

The word also means a critical turning point or dividing line when something important changes. The invention of the printing press was a watershed moment in history because it transformed how people shared information. A watershed event in your own life might be moving to a new city or starting middle school. Afterward, things are different in significant ways.