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River Features: Holes part 1

Updated: Jan 5, 2023

Video: a ledge hole on Canton Creek, Oregon. A tributary of the North Umpqua River, around 8k. My kayaker friend was side surfed here for a long time!

When I began rafting I had the hardest time telling crashing waves from holes. I would look down a rapid full of whitewater, and all I could see was white. I of course then would end up in every hole because when I got close enough to spot it, I would stare at it and plop right in! You go where you look, just like mountain biking and skiing. Without knowing what you are looking at, it will be difficult to make a decision on a route to take. So I started asking questions, I'm the kind of person who loves to categorize info! But I need to know what a feature is called so I can ask questions about it. I've learned not only to tell apart a crashing wave from a hole, but that they can both be versions of the same essential feature (an underwater rock or shelf) at different flows. Now I'm looking for holes I can hit for fun, (pleasure holes) and looking out for holes that will munch me (stoppers). In this 3 part series on holes, I will cover the basics we have all heard, the anatomy and types of holes, and some tips and techniques for running bigger holes - when you are ready to.

Photo: that same ledge hole in Canton Creek. Where are the best routes here?

What is a hole? Holes are caused where the downstream current dives to the bottom of the river over a rock or ledge and the void left behind the rock is then backfilled aggressively with surface water moving upstream to fill in the low pressure/ low elevation area. Steep crashing waves are also commonly referred to as holes when the backwash reaches the bottom of the trough. We will look at some types and features of holes later. Many holes are unfriendly places where water is recirculating on a vertical plane and the downstream current in a hole is plunging towards the bottom of the river before exiting downstream, creating a hazard for our boat by which we can be surfed or flipped if we don’t clear it. Here is the classic diagram we have all seen of the kind of hole you find behind a pour over.

In reality, holes are very dynamic because the rocks shaping them are all different shapes and sizes. The area of a multi faceted hole to look out for is the pocket, the sticky part that can hold anything floaty. Like an inflatable kayak.

Photo: Breitenbush River, Oregon. I ended up poking it with a stick to free it from this hole.

How are holes formed? A hole is created by an obstacle in fast current, let’s say a boulder. The same boulder can create different features at different flows. It could be a rock at low flows or a pour over at moderate flows. At high flows, that same boulder can create a hole or wave on the downstream side of it depending on the speed of the current and the depth of the rock. The speed of the current can be accelerated by volume of flow, compression/obstruction, gradient, and bends.

A Progression of features: Rock to wave.

1. Rock: Imagine you look downstream and you see moderately fast current entering the top of the rapid in a river wide downstream V, then splitting around a large boulder, which creates an eddy behind it and a pillow in front of it. Current then dissipates into a pool below. This sounds like an easy class II rapid, with one clear obstacle: a boulder. Our flow is a little low for this streambed, let’s say 1,000 cfs.

2. Pour Over: Now imagine that same rapid with twice as much volume; with 2,000 cfs the downstream V accelerates quickly, lateral shoulders feeding into the crest of the V wave. The current rides several feet up the huge boulder, forming a mound, and spills vertically over the backside of the rock as well as around the sides or shoulders. Instead of a rock, now with more volume we have a pour over. Is the backside going to be an eddy or a hole? At some point the amount of water spilling vertically over the back of the pour over will have enough volume driving towards the bottom of the river to create an obvious hole behind the pour over. Typically, a hole created by a single rounded boulder is smiling, a term we use to mean friendly based on the shape (more on types of holes later).

If there is no water cresting over the rock yet, then we still have an eddy. Somewhere in between is a grey area of opinion between hole/ eddy. With only a little water cresting the rock, or in a pour over with a sloping backside, often a weak hole forms that can be used like an eddy. Whether or not a pour over/ hole combo is a friendly feature or a hazardous one depends on retentiveness and scale of the feature compared to the boat, as well as the boaters skill set.

3. Wave: Our same rapid now has three times the flow. At 3,000 cfs the boulder is entirely submerged and a wave forms just slightly downstream of the rock. As a pour over transitions to a wave with increasing flow, at first the wave is very close to the rock that forms it. A bulge is present from the subsurface water compressing over the top of the boulder. After the bulge, the current dips down into a trough or low point and then ramps back up to the crest. At even higher flows, the wave forms further downstream of the underwater boulder and grows in height. As flow increases, waves become less predictable and more chaotic, often crashing and surging. Waves created by underwater rocks can be identified and picked out of a wave train because they stand in place and have a different shape than the surrounding waves in deep water. Being able to pick out a wave formed by a rock can help boaters avoid hitting the shallow rock.

Photo: 4 different approaches to run "Boat Eater" Illinois River, Oregon at moderate flows

Wave or hole?

One notable difference between a crashing wave and a hole is that the wave will flush a swimmer or boat after a flip. A hole could recirculate that swimmer. Riding the current into the face of a crashing wave, the current going under the backwash is exiting at surface level on the backside of the crest. In a hole, the current diving under the foam pile is going toward the bottom of the river, and when it resurfaces on the boil line, is then feeding back upstream. In a retentive hole, only the deepest current is exiting downstream of the boil line. Holes are retentive and waves are flushing.

Photo: Boat Eater at moderate flows, the crashing wave forms next to the midstream rock with lateral shoulders feeding into it.

The difference between a hole and a wave is a progression; the cross section of that progression shows the downstream current becoming more horizontal with more water as the feature transitions from hole to wave. Being able to differentiate a hole vs. a wave will help boaters decide the risk level involved with the feature. There is grey area among the terminology both regionally and within different paddling communities. Other terms used for holes include hydraulic, reversal and old school keeper or stopper. For our purposes, we will call a crashing wave a hole when the backwash creates a steep foam pile that reaches the bottom of the trough on the upstream face and the feature is large enough to pose a threat to our boat.

Photo: Boat Easter at high flows, the feature has moved downstream and forms more of a wave than a hole. The hole has become "washed out" but the lateral wave on river right has become it's own feature to avoid.

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