Friday 23 July 2021
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Holes and Waves

Holes and waves are hydraulics on a river that are created by water running over different obstacles and drops in gradient on the river bed, which has an effect on the current and river flow.


A hole is a feature created by water flowing over an obstruction on or near the river surface. Similar to an eddy the water flows over the obstacle creating a gap that needs to be refilled. Naturally the river refills this gap, in this case by folding back on itself and flowing back upstream, creating a continual recirculating flow of water.

Depending on the size and shape of the hole they can be great places for kayakers to play or extremely dangerous. Some holes can be can hold a kayak and in some cases a swimmer indefinitely.

Freestyle and play boating is the name given to skill of surfing and throwing tricks in friendly holes and on surf waves (see below).

How to recognise a friendly hole

There is a fine line in determine a safe and friendly hole. Holes come in a whole variety of different sizes, shapes and types. Here is our staring guide to help you begin to recognise the difference between friendly and unfriendly holes.

1. One of the key factors we can look for to determine if a hole is friendly or not is its shape. We can look to see if it is smiling or frowning which will give us a good idea if it is possible to get out of in our kayak.

A smiley hole has edges that curve downstream. A smiley hole is great because it gives us defined exit points.

A hole that frowns upstream tends to want to hold on to us. As you travel to the edge to get out it tends to push you back to the middle.

2. Another factor is the length of its boil line.

The strength or retentiveness of a hole can also be gauged by its boil line. The boil line is the point that separates the water that is exiting the hole and the water that is travelling back upstream into the hole for another ride. The boil line is located downstream of the seam (which is the point where the foam pile and the oncoming green water meet). The distance between the boil line and the seem will give you an idea of the strength of the hole. Generally the further apart they are the stronger the hole. Two to three feet is can often be a nice friendly hole but six, eight, ten or even fifteen feet will mostly tell us that we don’t want to go in there as that is way too powerful a hole.

3.  Your experience and comfort level.

The more experienced you become on the water the more you will be able to recognise the characteristics of different river features including holes. Also the better you will be able to cope with surfing or being stuck in a hole. However experienced you are though, a dangerous hole is a dangerous hole. If it is a powerful frowning hole or a hole with a large boil line, even the most skilled boaters in the world will not be able to get out.

If your in any doubt at all, avoid it. Its better to be safe.

Top Tip

The size of a hole does not necessarily determine how safe or friendly it is. Some small holes can be more dangerous than bigger ones, it all depends of the shape and dynamics of what has made the hole.

If you are unsure about whether a hole is safe or not. Ask a more experienced paddler for their advice. If you are still unsure avoid the hole and look for a smaller friendlier ones elsewhere.

It is always best to seek advice from more experienced paddlers who know the section of rivers or holes and waves you are looking to run and play on.


Waves are formed in a similar way to holes. A change or obstruction on the river bed again kicks up the water and breaks its normal flow causing it to ramp up and make a wave which sometimes will also fold back on itself. Depending on the flow of the river the same underwater feature can create a wave or a hole. The main difference between a wave and a hole is that a wave will often have a smoother green face to it and the water will be less of a recalculating hydraulic flow than a hole. That said some waves can have a foam pile (a section on it that folds back upstream) similar to a hole. Like holes waves can be awesome places to play. There is something very special about the feeling you get from surfing on a sweet wave.

Surfing is the name given to the process of catching a wave and moving / crusing around. It is achieved when you park your boat on the green face and use the power of the wave to allow you to carve back and forth across the face of the wave. Its the same principle used by surfers out on the ocean just in this case it all happens on a fixed point on a rapid as the wave stays in the same point and doesn’t travel along with the flow.

As well as proving us with hours of surfing fun, waves can also be used in other ways, they can be used to slow us down so we can scout a rapid part way down or to help us cross fast sections of flow.

Check out the play boating section to find out everything you need to know about surfing waves and holes.



Holes on the river

A big hole on the Ottawa River. Picture by Jez