Kayaking: Gurney's law of fast flows

Steve Gurney. Photo / Supplied

Steve Gurney is a New Zealand multi-sport legend, best known for winning the Coast to Coast race a record nine times, totally dominating the event from 1997 to 2003 when he won seven in a row. He has years of kayaking experience on the South Island's extensive braided river networks and knows more than a few tricks choosing a good line on a river like the Waimakariri that is forever changing.

The Waimakariri is a braided river (very rare in the world) and requires a unique skill to navigate for a fast trip. A braided river has numerous shallow rapids spread over a wide, pebbled bed. It meanders, splits into multiple channels which rejoin at different places.

The biggest challenge for beginners is avoiding a channel that runs out of water, requiring an arduous (and embarrassing) walk back to the main channel. Choosing the fastest channels can be the difference between first and second place in competition.

There are four rules I teach kayakers looking to paddle braided rivers faster.

(1) Look ahead: look as far ahead down the river as you can, at least a few hundred metres, to pick the major water flow.

(2) Biggest flow at junction: the biggest flow or biggest volume will move the fastest, and is also likely to be deepest. It's tempting to take a short cut, a shallower and skinnier branch, but invariably it's slower or sometimes runs out.

If it looks like your two options are of equal volume, two rules should guide your choice.

(3) Gravity rules: gravity plays a big part in determining where water flows. So if you've been looking ahead as in (1), you'll be able to figure out which channel ends up lowest, and hence where there will be bigger and faster flow. The higher channel will likely run out of water.

(4) Remember history: if you previously chose the right of two channels, there must be a channel somewhere on your left which will eventually rejoin the main river, or another channel. Statistically (and by the Law of Gurney) if you then get a choice of equal flows, take the left as it's very likely to meet the left flow that you previously didn't take.