Sierra Canyons Preview: Waterflows

This is a preview from the intro for a guidebook I’m hoping to have out sometime in 2021 on technical canyons in California’s Sierra Nevada. It’s in no way meant to tell you that you should or should not make any sort of choice. Also, if you’re keeping track, I am saying LITERALLY NOTHING related to crossing large swift water rivers, which is a completely different activity. 

Waterflows:

The canyons in the Sierra are not as narrow, deep, twisted or carved as the sandstone slots of the Colorado Plateau, however many people find them just as awesome, and worthwhile. Along with crystal clear emerald pools, the flowing water, beautiful as it is exciting and dangerous, entices us.

However, this is not to say that the canyons are not worthwhile at lower water. Even when the flows come down to a trickle, the pools often grow more brilliantly colored, and the jumps are easier to scout. Especially within social media circles, it has become common for people to use how high of flows they did a canyon at to measure their self worth.

Please allow me to tell you what your Instagram followers won’t: No one cares.

Just go play in the canyons. Enjoy the scenery. Have fun. Or, if there’s too much damn water, do something else. There are other canyons, or if that fails, there’s good breweries scattered about the Sierra as well.

I’m going to do the best job I can at helping you to forecast and assess the waterflow of these canyons. That said, it’s not easy, and it won’t be perfectly accurate. Likely you’ll be able to find evidence to support doing the canyon, even if it’s way too high. Don’t let the fact that you really really want to do the canyon alter your judgement. Listen to everyone’s input. Don’t just follow a single person’s lead. Don’t die!

Interpreting cfs (Cubic Feet per Second):

The best way we have to look at swiftwater conditions in canyons is cfs. If you think of the cross section of a river, or say, the lip of a waterfall, this is the volume of water that passes over that lip every second. The great thing about cfs, is that (disregarding tributaries), cfs is constant along a waterway.

However, what we are actually concerned with is the force that can be exerted on our bodies. One cfs of water equates to 28.32kg/s of mass. Mass per second is still just a measure of flow. This actual force is calculated by multiplying that 28kg/s by the change in velocity. If we assume your body is stopped in the water, then your velocity is zero.

So more simply flow times velocity is the force of the river. This means at 1 meter per second velocity, and 1 cfs flow, we have 28kg m/s2 of force, which is 28 newtons, or 6.3 pounds. Say we have a canyon with 5 cfs, flowing through a rapid that moves water at 10m/s. That’s 6.3*5*10 (315) pounds of force!

Fortunately, most of the time you are not facing the entire force of the water. In particularly narrow canyons, you might face most of it. That’s why 5cfs can be a lot for a narrow slot. However, in a wide canyon, you might only be hit with 10% of the water, and only 10% of the force. So different canyons can work at different flows.

Let’s think of another example. Say the canyon is flowing at 20cfs. You start at a pool, where it flows at 0.1meters/second. The force of the river here is 12.6 pounds. It’s calm, and doesn’t look intimidating. However, you get to the same rapid, which because of higher flows would move very fast, say 15meters/second. The river now has nearly a full ton of force behind it! It’s a raging torrent, and something you probably want no part of. This is a common problem with looking at the flow of canyons. They appear tame, until they are not. This is why it is super important to have an idea of the flow of a canyon.

You may think, what if a canyon only flows at 2cfs, but the water moves at 50m/s!? That’s 600 pounds of force! Is cfs completely useless? No it isn’t. The water will not be going that fast. A frictionless freefall of 100 meters would provide a speed of 44m/s. Water is subject to friction, and the less water there is, the more it tends to disperse, and the more it is slowed by air. I’ve heard people discuss formulating and deciding on what kind of force you get from what height drop and what flow, but I don’t think this is particularly necessary or useful. I can think of one waterfall, in Garden Creek in the Grand Canyon, where the ~2cfs of water flows down ~25meters down a chute that keeps it from dispersing. The result is water current powerful enough to take your feet out from under you, which if you’re confident at rappelling, is pretty funny. So, unless you are in something like a cave, and find yourself forced to take all of 1-2cfs directly to the face, these flows typically will not pose seriously problems given effect use of swiftwater canyon techniques.

So I argue that knowing just the water flow, and the general width/nature of the gorge, we can get a good idea of what reasonable flows are. Specific features are necessary to take into account, mostly when pushing the upper limits of reasonable flow.

Here’s a quick chart for reference. Imagine we have 3 types of canyons: Narrow slots like you find in Utah, typical Sierra Canyons that tend to be ~6-10 feet wide, and larger river gorges likes the East and Middle Forks of the Kaweah, that are less steep and typically over ten feet at even their narrowest points.

Please keep in mind that none of this is definitive. Specific canyons have specific features that can be more or less dangerous at different flows. For instance, if I (person who is semi-experienced with swift water canyons) were descending a canyon I had little to no information on, I would look for the flow to be in the Low-Moderate range.

Flow Narrow Slot Sierra Canyon River Gorge
2 cfs Moderate: Might hit hard, but should not be problematic. Low: Should not cause issues, and be appropriate for less experienced swiftwater canyoneers. Very low: Likely manageable with very little swiftwater experience.
4 cfs Moderate-High: Expect to get pounded by waterfalls and potentially swim or work against strong currents. Low-Moderate: Might hit hard, but should not be problematic. Low: Should not cause issues, and be appropriate for less experienced swiftwater canyoneers.
6 cfs High: Force of water may be prohibitive for rappelling. Strong swimming required to deal with eddies and hydraulics. Moderate: Rappelling directly in the current could prove difficult. Low-moderate: If channelized pourovers are avoided, things should still be easy.
10 cfs Extreme: Rappelling directly in flow, swimming through hydraulics, and avoiding being swept over drops will all prove risky. High: Force of water may be prohibitive for rappelling. Strong swimming required to deal with eddies and hydraulics. Moderate: Plan routes carefully at falls to avoid the strongest flows. Experience with swiftwater necessary.
15 cfs Don’t do it! Very high: Rappels should be directed out of flow. Risk of strong hydraulics and being swept over falls is very real. Moderate-High: A little more caution and experience required, but likely still manageable.
25 cfs Absolutely not! Extreme: Safety lines necessary above drops. Hydraulics and waterflow of falls must be avoided. Most Sierra Canyons are not descendable at these flows. High: Most often doable, but strong swimming by all participants, careful route planning, and swiftwater experience are required.
50+
cfs
Certain death No! Extreme: Bypasses will need to be found for most drops. Strong swimming to cross current is likely necessary. Maybe doable by strong swimmers, experienced in swiftwater, in canyons they are familiar with.

 

Anyway, there are my thoughts on people and water moving through canyons in lovely harmony. I am by know means an expert on fluid dynamics, so if anyone wants to correct my physics, please do!

Coming next…? How to estimate cfs in a creek. It’s easy and it’s important! How much do you think is coming down the waterfall at the top of the page? Any guesses? 

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