Rapids are sections of a river where the water flows very fast and turbulently over rocks and through narrow passages. The surface churns and froths white as the current rushes downhill, creating waves and swirls that make the water look like it's boiling. Rapids form when a river's smooth flow gets disrupted by obstacles like boulders, sudden drops in elevation, or a narrowing of the riverbed.

Rivers with rapids are classified by difficulty, from Class I (easy, with small waves) to Class VI (nearly impossible and extremely dangerous). Class II or III rapids might be fun for families in rafts with guides, while Class V rapids challenge even expert kayakers. The Colorado River's rapids through the Grand Canyon attract thousands of adventurous rafters each year, while the rapids on Idaho's Salmon River offer thrilling rides through wilderness.

If you've ever watched a mountain stream rush over rocks after heavy rain, you've seen small rapids in action. The roar of the water, the spray in the air, and the sheer force of the current make rapids both exciting and worthy of respect.