While the majority of these side canyons are dry, when they flood, huge amounts of water can move down them as flash flood s. 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 as single dramatic events, these rocks reach the main channel of the Colorado at their confluence.
This clogs up the river, creating a dam across the river bed and narrowing the channel. All but a couple small rapids in Grand Canyon are made by these side canyon drainages. As the water flows over the obstruction of boulders and through the narrower channel created by these natural dams, it picks up speed.
Some waves within a rapid are made by water going over boulders near the surface, but most waves in the main channel are caused by fast water moving steeply downhill in a narrowed channel 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 or a narrowed channel, it has nowhere to go but up, creating what is called a hydraulic jump wave.
Your guides will call these standing waves or tail waves. Usually, this is where your guide will place your raft for the best ride in the rapid! A raft entering a rapid will float agonizingly slow across the lake-like pool above the rapid to the edge, or the lip of the rapid. Generally, the guide will place the raft in the current of 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 over this smooth tongue at the lip of the rapid, until it hits the first of a series of waves that the V shaped tongue of smooth fast current will invariably slam into.
The largest rapids in Grand Canyon are known for having huge waves. While exploring the Grand Canyon on your next Colorado River trip, you will experience the blood pumping excitement of world famous rapids like Soap Creek, Crystal, and Lava Falls. Rapid is a term you will hear over and over on your trip, but you will also hear a lesser known term referring to a different type of whitewater — a riffle.
What is the difference between a riffle and a rapid? First of all, whitewater sections on the Colorado River are rated differently than elsewhere. They are assigned a difficulty level on a scale of , with 10 being the most challenging. Despite their baby status, riffles usually have a rating of about 2 and can still take you for a decent ride. Obstacles form the most prominent features in rapids, such as waves, holes, and pourovers. Rapids form in a river anywhere the geology allows for gradient, constriction, or obstacles.
It is common to find rapids in steeper sections of the river, because the increased gradient causes the water to flow faster. The type of rock that makes up the riverbed also plays a huge role in the formation of rapids. When the river reaches a layer of rock that is more difficult to erode, it forms narrow channels. In these constrictions, you will typically find stretches of river full of whitewater. Another place rapids are commonly found is at the mouth of a large side canyon.
Massive flash flood events in side canyons rip up boulders and send them hurtling into the river, creating debris fields at the mouth of the canyon. These debris fields form technical rapids choked with obstacles. Straightforward, wide rapids with several clearly navigable channels. Some easy maneuvering may be required. Rapids have strong eddies and currents and moderately sized waves.
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