What does it mean for river rapids to be graded "2-3"?

Question

What do the numbers mean, and how high does the scale go?

Answer 1

Cut and pasted from the link below (the link talks about W. Va. rivers, but the scale applies):

"River Difficulty Scale

The class of any rapid may change as water conditions vary. In general, rapids get more difficult as the water level rises. Interestingly, a significant number of rapids on both the New and Gauley rivers in WV actually get tougher as the water level drops. Rapid classifications must be thought of as guidelines, not descriptions. In the fluid world of hydrodynamics, things change quickly. Good paddlers always consider the total environment not just the class of the rapids.That being said, here is the International Scale of Difficulty which rivers are classified:

Class I

Moving water with few riffles, small waves, few or no obstructions.

Class II

Easy rapids with small waves and wide, clear channels that are obvious. Some maneuvering required.

Class III

Waves may be high and irregular. Passages may be narrow and may require complex maneuvering. Obstructions are not life-threatening under normal circumstances. The route is clearly visible from the top of the rapid. Many of the New and Gauley's most entertaining rapids are Class III's.

Class IV

Very difficult water. Long, constricted, difficult passages that require complex maneuvering. Conditions make rescue difficult. Waves may be large, irregular, and breaking...capable of capsizing a raft. Life-threatening obstructions, such as undercut rocks, may be present. Routes are often not readily apparent from the top of the rapid. Any Class IV rapid is serious and requires the attention of everyone in the boat.

Class V

Extremely difficult and violent rapids with significant hazard to life in the event of a mishap. Rescue is very difficult and there is significant hazard to life. Waves may be huge, numerous, and breaking. Life-threatening obstructions, such as undercut rocks and "keeper" hydraulics may be numerous. Many Class V rapids are relatively long stretches of whitewater that require constant maneuvering. This requires intense teamwork between the crew and the Guide.

Class V+

A commercial designation, this represents the absolute limit of commercially run whitewater. Huge waves and hydraulics and many obstructions which can be life-threatening. Rescue conditions are very difficult. For teams of experts only. Class V+ rapids on the Upper Gauley in WV include Insignificant, Pillow Rock, Lost Paddle, Iron Ring, and Sweets Falls.

Class VI

Nearly impossible and very dangerous. Can be run only by teams of experts with significant danger to life. There are no commercially-run Class VI rivers. Many of the steep creeks in West Virginia, such as Mann's Creek and the Lower Meadow river are true Class VI runs."

Answer 2

the scale goes to 5 the higher the number the rougher the river rapids are gonna be