Tide Alert: Caution is Advised

June-July 1999

This is an article from WaveLength Magazine, available in print in North America and globally on the web

by Doug Lloyd

In most of BC's waters, tidal movements can be predicted with reasonable accuracy using tide and current tables. Within the complex maze of channels and islands of BC's inland waterways, however, there are other influences that must be accounted for besides the cyclic nature of heavenly bodies.

Winds can generate surface currents that may prolong an ebb or flood, especially in the narrow channels such as those of Johnstone Strait and Discovery Passage. Even in the wider Queen Charlotte Strait, winds can enhance or reduce the strength and duration of ebb and flood currents.

Fresh water runoff from rain and snow melt may also affect predicted tide values, or worse, flow atop the surface while denser ocean water subducts underneath, leaving the uninformed kayaker drifting in a direction that contraindicates predicted current directions. November and December are usually the worst for rain water run-off. May and June (as well as July on the North Coast) are the worst for snow melt. With a 105% snow overload this year, caution is advised.

Caution is also advised when attempting long crossings of these straits. Divers report currents running in opposite directions as they descend the water column. Estuarine circulation and other considerations produce strong outflow currents seaward, often to a depth of 100 meters, and can weaken or halt flood currents at the surface for days resulting in a persistent ebb, or ebb bias, confounding your best calculations.

Individuals who utilize software to generate computer models of tidal movement would be well advised to exercise caution too, as the data supplied is based on the average of the water column, and may not give you accurate information. You can call Bodo de Lange Boom at the Institute of Ocean Sciences in Sidney, if you have further questions: 250 363-6358.

Doug Lloyd is a frequent contributor to Wavelength