A Glossary of Commonly Used Grand Canyon Rafting Terms
Below is a list of the commonly used Grand Canyon rafting terms you’re likely to hear on our rafting adventures. You’ll probably hear your guides chatting about the breaking waves, debris flows, water volume, holes and many more of these features and terms.
Big Water: Large Volume, fast current and big waves, often accompanied by huge reversals and extreme general turbulence.
Breaking waves: Where the top of the wave has collapsed over itself, creating a white, frothy look.
Confluence: The point where two or more rivers meet.
Confused Water: Typically seen at larger eddy fences (see definition below). Where the shear zone between the two currents can be quite broad with water swirling about erratically with no definitive line between the two differing currents.
Cubic Feet per Second (CFS): Defines the volume of water one foot deep and one foot wide flowing a distance of one foot in one second. One CFS is equal to 7.48 gallons of water flowing through a cubic foot area each second.
Debris Flow: Fast moving, liquefied landslides of mixed and unconsolidated water that look like flowing concrete.
Eddy: Sometimes referred to as a backwater. It is where river current flows past an obstacle, such as a rock bar, delta or makes a bend and the current must return upstream to fill the void.
Eddy Fence (or Eddy Line): The area where the upstream eddy current moves past the downstream river current. Creating a shear zone between the two currents going different directions. Large eddy fences can have whirlpools.
Flash Flood: When a large amount of water (usually caused by rain) flows through an area. This typically occurs in the side streams, tributaries and slot canyons in the Grand Canyon.
Flat Water: Also called calm or smooth water. Where the river is calm and usually slow moving, with no waves of any sort.
Gradient: The “steepness” of a river, measured in feet of elevation loss per mile of river. Average gradient is the average over the total length of a section of river.
Hay Stack(s): Waves that are large, but usually do not have much breaking water reversing at the crest, but they can be steep.
High Volume: A large amount of water passing through an area of the river.
High Water: River flow above an expected average. It makes the currents faster and some rapids easier. Other rapids become more difficult.
Hole(s): Where water flowing over a rock or other obstacle flows down, then back onto itself in a frothy foam of whitewater.
Horizon Line: Usually indicative of a fall or steep drop. There is a line to run, but the route, if there is one, is not apparent. This means the guides will get off the rafts and start scouting the rapid if they are not familiar with it.
Hydraulic(s): Also defined as waves or holes. On a river, they are created by water running over different obstacles and drops in gradient on the river bed.
Hydraulic Jump Wave: The rise of water level, which takes place due to the transformation of unstable shooting flow (super-critical) to the stable streaming flow (sub-critical flow). It frequently occurs in a canal below a regulating sluice, at the toe of a spillway, downstream of a narrow channel or at the place where a steep channel slope suddenly turns flat. On the Colorado River in Grand Canyon, it occurs downstream of a narrow channel, or at the place where a steep channel slope suddenly turns flat.
Keeper: A hydraulic reversal capable of trapping a raft or sometimes people for long periods (see holes above).
Lip: The name of the entrance to the rapid. The instant you go over the edge of the horizon line (see definition above), and head down into the rapid, you are going over the lip.
Standing wave(s): Often used interchangeable with haystacks and tail waves (see definition below).
Tail waves: Generally, the smaller standing waves at the end of a rapid.
Technical rapid: Requiring moves around rocks or hydraulics to successfully navigate a rapid. The more moves, the more technical a rapid.
Tongue: The smooth “V” of fast water found at the head of rapids.
Volume (high or low/big or small): Terms used for describing the amount of water flowing through a given channel.
Wave Train: A series of standing waves in close succession.
Whitewater Rating or Class: Whitewater is classified on an international scale of I-VI, with VI considered extremely dangerous and not runnable by rafts. In the Grand Canyon rapids are rated 1-10 and are compared against each other. However on the International Whitewater I-VI Rating or Classification, the most challenging 9 and 10 Grand Canyon rapids fall into Class IV rating on the International Scale at normal water flows. Learn more at American Whitewater Association.