Grand Canyon Rapids
Grand Canyon rapids are known for being big, wet, and intimidating, but few Grand Canyon rapids are referred to as technical. Technical rapids require making a move, or change in direction, when navigating a rapid. The more moves made, the more technical a rapid. Most Grand Canyon rapids only require the guide to enter a rapid at a fairly precise location and float through the wave train. However, there are a few rapids that are considered technical, and require one or two moves. These few technical rapids, typically have breaking waves, holes (also known as hydraulics or keepers), and occasionally rocks or cliff walls that need to be avoided.
Most commercially rafted whitewater rivers have ‘gradients’ between 25 and 50 feet per mile. The average gradient of the Colorado River through Grand Canyon is relatively low, at just under 8 feet per mile. Comparable to one of the steepest sections of the Missouri River, averaging 9 feet per mile, and not considered to be a whitewater segment of river. The low gradient of the Colorado River through Grand Canyon would lead one to believe it is a ‘flat water’ river; and in reality almost 95% of the river is flat water. So why are there any Grand Canyon rapids?
The answer lies in the numerous side canyons and drainages of the Grand Canyon, which have extremely high average gradients, in many cases thousands of feet per mile. While most of these canyons are dry the majority of the year, when they do flood, huge amounts of water can move down them in flash flood scenarios. The steepness of these drainages increases the velocity of the water and its power to move giant boulders and massive amounts of solid material. Sometimes landslides occur during the flooding events and combine with the water in the side canyon to make slurry of mud that can actually float giant boulders weighing many tons. When this occurs it is called a ‘debris flow’. Over time and occasionally even in dramatic events, these rocks reach the main channel of the Colorado where they join at their ‘confluence’, which effectively clog up the river, create a dam across the river bed and narrow the channel. Almost every rapid in Grand Canyon is made by these side canyon drainages.
When this occurs, the river backs up above these choking boulder dams, creating a slow ‘pool’ above the rapids and a ‘horizon line’ over the drop when looking downstream. A raft entering a rapid seems to float agonizingly slowly across the pool above the rapid to the edge, or the ‘lip’ of the rapid. Generally, the guide will place the raft in the ‘tongue’, a ‘V” shaped section of smooth water that indicates the deepest part of the channel through the obstruction. Suddenly, the raft will plunge downhill over this smooth tongue at the lip of the rapid, until it hits the first waves that the V shaped tongue will invariably slam into.
As the water flows down over the obstruction of boulders, making the dam and through the narrower channel, it picks up speed. Many waves within a rapid are made by water going over boulders near the surface. Most of the waves in the deeper channel are caused by fast water heading down hill and slamming into the slower water below the rapid where the river bed begins to return to its normal low gradient. When the fast water hits the slower current it has nowhere to go but up, creating what is called a hydraulic jump, which is the term used in hydraulic science http://en.wikipedia.org/wiki/Hydraulic_jump. In the Grand Canyon and other rivers these are commonly called standing waves or tail waves by your guides. This is usually where your guide will place your raft for the best ride in the rapid!
The Colorado River is considered a ‘big water’ river meaning it has a high volume of water, generally much higher than technical whitewater rivers. The ‘cubic feet per second (cfs)” of the Colorado through Grand Canyon averages about 12-15,000cfs in the commercial rafting season, but has flowed as low as 5000cfs and as high as 25,000cfs in that same time frame on some years. Pre-dam the river regularly ran as high as 100,000cfs in the spring and some years substantially more. Every few years, due to scientific studies, there are temporary dam releases that can be between 35,000cfs and 46,000cfs. In 1983 the river though Grand Canyon had a peak flow of approximately 100,000cfs. This high flow was due to an exceptionally big snow pack in the upper Colorado River basin of Colorado and Wyoming, as well as a miscalculation by the government agency regulating the Glen Canyon dam releases. What all this cfs talk means, is that the average high flow of water going down the river makes for BIG waves and BIG fun when going over obstructions in the river!
Badger Rapid at Mile 8, which is the first major rapid through Grand Canyon. Large side canyons on either side of the river have pushed large amounts of boulders in the channel. Note the V shaped tongue in the middle at the top of the rapid. This is where rafts will enter Badger Rapid and continue in a fairly straight line to the smooth water at the end. Check out Google Maps or Google Earth to see more geomorphology of Grand Canyon rapids.
Glossary: For descriptions of rafting terms used in this article.
Big Water: Large Volume, fast current and big waves, often accompanied by huge reversals (see definition below) 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
Debris Flow: Fast moving, liquefied landslides of mixed and unconsolidated water and that look like flowing concrete
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
Gradient: The “steepness” of a river, measured in feet of elevation loss per mile of river
High Volume: A large amount of water passing through an area of the river
High Water: River flow above an expected average, makes the currents faster and makes some rapids get easier, others become more difficult
Hole: Where water flowing over a rock or other obstacle flows down, then back onto itself in aneruption 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
Keeper: A reversal capable of trapping a raft for long periods
Lip: The name of the entrance to the rapid. The instant you go over the horizon line (see definition below), and head down into the rapid, you are going through the lip
Standing wave: See haystacks. Also interchangeable with tail waves (see definition below)
Tail waves: Generally, the smaller waves at the end of a rapid
Tongue: The smooth “v” of fast water found at the head of rapids
Wave Train: A series of standing waves in close succession
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