Rapids are a portion of river with especially turbulent water due to an increase in gradient, a narrowing of the available river space, an increase in the flow of water, and obstacles such as rocks or tree logs. Rapids may vary in their degree of danger to river-going craft. In fictional worlds, rapids may be paired with a waterfall to create a particularly treacherous stretch of river. They may occur within a river gorge making escape difficult. There may be portages to circumnavigate the danger.

Rapids may be categorized into classes ranging from I to VI. Class I rapids are easy to navigate while class VI rapids pose a significant threat to life.