Secrets of the St. Lawrence

The wind and the waves (PDF; 40 KB)

Sept-Îles

- "We have to cross to Anticosti today, or we'll have trouble tomorrow. They're calling for 30 knot winds tomorrow and the sea will be too high for my liking. Sailing is a lot of fun, but you need strong nerves."

Fetch

If there were never any wind, the St Lawrence would be a gigantic mirror, rising and falling with the tides. But that is not the case at all.

The St Lawrence is a vast surface that can be whipped up into violent seas depending on the direction, duration and speed of the wind.

Fetch is the distance over which the wind has been blowing from the same direction. The longer the fetch, the higher and longer the waves. After 12 hours at the same speed, though, the wind has almost no effect on the waves, except that it may cause them to lengthen, distance permitting.

Since the fetch is limited on the St Lawrence, the waves cannot lengthen as much as they do out in the open ocean, so they often become very steep.

In July and August, the waves are rarely higher than 3 metres.

Swell and wind waves

Waves that have been formed elsewhere or before the wind changed direction are called swell. The swell can be an indication of approaching winds.

If the waves are flowing in the same direction as the wind, however, you are looking at wind waves. If the wind should shift, you will encounter cross seas.

Fetch: 50 nautical miles
Duration: 6 hours