Whitewater canoeing is the sport of paddling a canoe on a moving body of water, typically a whitewater river. Whitewater canoeing can range from simple, carefree gently moving water, to demanding, dangerous whitewater. River rapids are graded like ski runs according to the difficulty, danger or severity of the rapid. Whitewater grades (or classes) range from I or 1 (the easiest) to VI or 6 (the most difficult/dangerous). Grade/Class I can be described as slightly moving water with ripples. Grade/Class VI can be described as severe or almost unrunnable whitewater, such as Niagara Falls.
The canoe (or just 'boat') used in casual whitewater canoeing is different from those used in whitewater racing. Traditionally, canoes were made of tree bark, sewn with tree roots and sealed with resin. Early whitewater boats were made of wood followed by aluminium and later fiberglass or kevlar, followed by more exotic composite materials including spectra, vectran and carbon fiber. The various composite materials are still preferred for racing due to the lightweight, but most modern recreational whitewater boats are typically rotomoulded from a tough plastic or molded from a plastic laminate called Royalex which is an ABS plastic that is slightly flexible and very durable, if easily scratched but repairable using either plastic welding or a variety of patch bonding techniques. Boats can range in size from barely long enough to hold the paddler (around 6 ft (1.8 m) long), up to 12 ft (3.7 m) or longer for solo boats and typically 14–16 ft for tandem boats.