Headwaters are the streams and springs where a river begins. Picture a mighty river like the Mississippi, wide enough for huge barges to travel on, then imagine following it upstream, past smaller and smaller tributaries, until you reach the trickling creeks high in the mountains where all that water first emerges. Those are the headwaters.

Rivers flow downstream toward the ocean, so their beginning point is at the head of the system, up in the hills or mountains. The Amazon's headwaters start as tiny streams high in the Andes Mountains of Peru, then flow thousands of miles across South America. The Missouri River's headwaters begin in Montana's Rocky Mountains.

Headwaters are crucial because they're often the cleanest, coldest part of a river system, providing habitat for fish like trout that need pristine conditions. They also feed everything downstream, so protecting headwaters means protecting entire river systems. If a factory pollutes the headwaters, that pollution can travel downstream to affect everyone and everything below.

You might hear people say someone is “at the headwaters” of a movement or idea, meaning they were there at the very beginning, when it first started flowing.