by John Lull
(editor’s note: This week, Tsunami Ranger and earth scientist John Lull ‘splains the difference between river hydraulics and water dynamics present in ocean rock gardens)
At first glance the whitewater in ocean rock gardens looks very similar to a whitewater river. You’ll see waves, aerated rough water, rocks, currents, and swirling chaotic conditions. They share some characteristics, but are actually very different environments. If you are a river kayaker, many of the skills you use in the river will transfer to paddling in ocean rock gardens, and vice versa, but you’ll need to understand and learn how to handle the different dynamics. I’m going to outline some of the similarities and, more importantly, the differences between river and ocean whitewater. Probably the closest features to a river in the marine environment are tidal rips that form in bays and constricted passages between the open sea and inland waterways. But for this article I’m going to focus on ocean rock gardens.
This will be an overview. We can get into more details in the comments if you wish.
Similarities between rivers and ocean rock gardens
Rough water and waves are present in both river rapids and ocean rock gardens. So you’ll need all your rough water kayaking skills, including boat control, edging, balance, bracing, swimming skills, and rolling (especially in a closed deck kayak). In rock gardens, currents are induced by surging waves and to some extent they behave just like the current in a river. There is an important difference that I’ll discuss below. These currents move through and between rocks, as they do in a river, and in both cases you can ride the current. Good maneuvering skill, especially a good draw stroke, is necessary in both situations.
In a river, eddies can be used to stop, hang out, rest, and even scout the next rapid. You can also use them as launching points for surfing a standing wave, or making your next move. Similarly, in rock gardens there are often ‘safe zones’ behind rocks (on the shoreward side), where you are relatively protected from the waves and can rest, scout the area, and time your next move. These zones are not the same thing as an eddy, but can be used in a similar fashion.
And now for the most important similarity: They are both fun and exciting places to mess around in a kayak!
Differences between rivers and ocean rock gardens
Waves are a primary feature in both river and ocean whitewater, but the nature of the waves is different. In the ocean, the wave form is moving through the water and the water is only moving up and down in a circular fashion as the wave passes through. In a river, the wave form is stationary and the water is moving through it, as a current. This has important ramifications for a kayaker. If you surf a wave in the river, you’ll be standing still (facing upstream). You might move back and forth on the wave face, but you won’t be going anywhere. When surfing a wave in an ocean rock garden, you’ll be moving with the wave. You’ll have to be aware of where that wave is taking you!
The waves in a river stay in one place and don’t change much, at any given flow rate. The waves in ocean rock gardens constantly change in both size and power, and they can even arrive from different directions. This wave variation in the ocean might be the most important difference. The result is constantly changing dynamics, depending on the size of the waves. When you scout an ocean rock garden, you have to watch several sets of waves to see what happens, especially what happens when the larger wave sets move through. Seemingly benign conditions in a rock garden can suddenly explode into a surging cauldron when a wave set arrives. The action will intensify with each wave in the set.
The current in a river is constant and moves in one direction (with the exception of an eddy, where the current moves in the opposite direction). The wave-induced currents in an ocean rock garden are not constant and they move back and forth, first moving into the rocks, then reversing and moving back out. These currents are actually wave surge and they tend to ricochet around, between and over rocks like a berserk pinball machine, unlike river current which is more channelized.
A very important result of the different type of current in a river vs. ocean rock garden is how it affects a kayaker when broached on a rock. A river current will tend to trap you against the rock if you happen to get broached sideways, so you need to lean into a rock in the river and present your hull to the current. Otherwise the current will flip you and trap you against the rock. If the wave surge in an ocean rock garden carries you sideways into a rock, you lean away from the rock and present your hull to the rock. The surge will reverse direction and flow back off the rock, taking you with it. And your hull will take the impact instead of your body.
A pour-over in the river might look the same as a pour-over in the ocean, but there is one important difference. In the river, when water flows over a ledge, a ‘hole’ or ‘hydraulic’ forms on the downstream side and can trap you. When this happens you have to work your way out by moving to the side of the hole to escape. In the ocean, a similar hydraulic may form at the bottom of a pour-over, but if you get trapped, it will only be temporary and you’ll be released, or pushed elsewhere, when the wave recedes.
Paddling in whitewater is fun and challenging, whether you paddle rivers or ocean rock gardens. Many kayaking skills transfer from one regime to the other, but you’ll have to adjust to the different water dynamics and learn to adjust to either environment. The river tends to be more of a steady-state situation, whereas the ocean is in constant flux and will change by the minute.
What is your take on John’s description of river vs ocean whitewater? Please post your comments or questions below this essay.