Reading the Water
Rivers have a magic way of healing, transforming and inspiring. From the calmest riffle to barnstorming haystacks, rivers always have rapids. This is how the river transfers energy. Monte Tillinghast grew up on rivers and has spent most of his adult years rowing the Grand Canyon of the Colorado. He knows a thing or two about rapids and has a gracious way of describing them. We talk with Monte about how and why rapids form in rivers.
Monte Tillinghast first went down the Grand Canyon in 1965 as a 3-year old. Almost 60 years later, Monte continues to row the Grand Canyon, principally for Grand Canyon Dories. A licensed pilot and engineer by training, Monte says the river has a way of instilling an inner-peace, a way of grounding, and a way of looking at things with clarity.
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Interview Excerpt - Roiling Rapids
Science Moab: Can you explain the concept of an eddy?
Tillinghast: Simply put, it’s just an area in a river where water is actually traveling the opposite direction in which the current your boat is headed. When the river drops, the current is going to speed up. As it does, it’ll sooner or later encounter an area that doesn’t have as much speed. Now you’ve got a situation where faster water is pushing under slower water. As a result, something’s got to give. You’re not going to compress the fluid when compared to a gas, so something in the form of hydraulics has to happen here and the faster water merely pushes the slower water out of its way. So this results in an eddy and you’ll notice that what separates the current going upstream versus the current going downstream is a line known as an eddy fence. It’s just a shear line where the water is changing direction.
Science Moab: How does the creation of these eddies relate to the gradient of the river and then to the rapids?
Tillinghast: The steeper the grade, the faster water is going to move. There’s a relationship there. So if you take this idea and go a little bit further with it, then if you steepen the grade, there’s going to be a point where this water is not doing much in front of water that’s moving faster, there’s going to be a point where something has to give there. In other words, an eddy is no longer sufficient to dissipate the amount of energy built up. What results is what some people refer to as a haystack. Some people call it a hydraulic, you know, some people call it a standing wave. When you pile a bunch of water up, there’s going to be a point where gravity says no more. Water will start falling down the backside of the wave. It will continue to do so below the river level until it creates a trough and at that point rebounds back upward. Wave heights beyond that point continue to dwindle. At that point, an eddy on one side or both sides of the current will likely occur. So grade has an effect on eddies and it also has an effect on waves if you increase the grade high enough.
Science Moab: What’s another way rapids can form?
Tillinghast: If you litter the river with a bunch of debris or rocks you’re probably going to have to have waves. If you’re in a canyon, you have cliffs where rockfall occurs. Side canyons, where tributaries join up with a river, are also typically where you have a rapid form. So if a torrent hits the side canyon, boulders, the size of houses, vehicles, etc, can make their way into the river. In severe cases, it can actually dam the river if you have a high enough flow in that tributary. This is called an obstruction. Upstream of the dam, the water will just continue to pile up until at some point, it breaches the dam. When you’re sitting at the top of a situation like that, you can’t see down the ramp of this form by virtue of the grade and the top of this rapid is going to be rocky by virtue of the dam. on the backside of the dam, water speeds up and as you get down toward the tail end of the ramp and haystacks, standing waves, etc, form by virtue of a faster current hitting water not going as fast in front of it creating these tail waves. So,with a lot of these rapids, you get a situation where up top it’s pretty rocky and then as you get down toward the bottom, these tail waves are not formed by rocks, but by a form of hydraulics.
Science Moab: So how about river constriction? How does that affect the waves?
Tillinghast: As far as the constriction goes, if debris doesn’t completely dam the river up, it can choke the river off. So in other words, you have a constriction that results from it. Another way in which you can have a constriction is a narrowing of the cliff walls. I’ll just mention Bernoulli principle here. One of the aspects of Bernoulli principle is the volume in has to equal volume out. When you have a situation where the river is constricted, you create a situation where water going into that constriction can’t go through at a velocity sufficient to achieve volume in equaling volume out, so something else has to happen. Above the constriction, water will just continue to pile up and at some point, with water at eight pounds per gallon, enough pressure is created to blast water through that constriction at whatever velocity needs to occur to achieve volume and equal in volume out. The weight of the water that’s being impounded above the construction creates a higher velocity of water. when the constriction subsides, or when things open up, this fast water is charging into slower water in front of it. So once again, waves are created as a result of a constriction. It’s a form of hydraulics. Textbook examples of that include Hermit rapid in the Grand Canyon and rapid number seven in Cataract Canyon.
