Flatwater- There is little or no current, and the river's surface is smooth and unbroken. paddling upstream is easy.

Quickwater- The river moves fast. Its surface is nearly smooth at high water levels, but likely to be choppy at medium levels and shallow at low water levels.

Class I- Fast moving water with a riffles and small waves. Few or no obstructions, all obvious and easily missed with little training.  Risk to swimmers is slight; self-rescue is easy.

Class II- Straight forward rapids with wide, clear channels which are evident without scouting. Occasional maneuvering may be required, but rocks and medium-sized waves are easily missed by trained paddlers. Rapids with waves up to three feet

Class III- Rapids with high, irregular waves which may be difficult to avoid often capable of swamping an open canoe. Narrow passages that often require complex maneuvering in fast current. Good boat control required: large waves or strainers may be present. Strong eddies and powerful current effects can be found, particularly on large-volume rivers.  May require scouting from shore. Group assistance may be required to avoid long swims.

Class IV- Intense, powerful but predictable rapids requiring precise boat handling in turbulent water.  Depending on the character of the river, it may feature large, unavoidable waves and holes or constricted passages demanding fast maneuvers under pressure.  A fast, reliable eddy turn may be needed to initiate maneuvers, scout rapids or rest.  Rapids may require "must" moves above dangerous  hazards.  Scouting is necessary the first time down.  Risk of injury to swimmers is moderate to high, and water conditions may make self rescue difficult.

Class V- Extreme.  These runs often exemplify the extremes of difficulty, unpredictability, and danger.  The consequences of errors are very severe and rescue may be impossible.  For teams of experts only, at favorable water levels, after close personal inspection and taking all precautions.

(*AMC River Guide descriptions)