Winds, Currents and Tides in the St Lawrence River

Winds, Currents and Tides in the St Lawrence River

The prevailing winds in the St Lawrence River are southwesterly. Although passing weather systems  have an impact on the wind direction, the funnelling effect of the river and surrounding mountains on means that in practice winds are invariably either southwesterly or northeasterly, with a heavy predominance of the former.

The predominate flow of the currents and tides is northeasterly,  with the ebb tide combining with the river current to give 8 or more hours of ebb flow at rates of up to 6-7 knots in places. In contrast the flood  is generally much weaker, often not more than one or 2 knots, and of much shorter duration. By the time one is half way between Quebec and Montreal, although the river remains tidal, the outgoing river current completely cancels out the flood flow. 

The net effect is that sailing out of the St Lawrence is an awful to easier than sailing in, and from the above description one might conclude that heading upstream is something only for the diehard masochist or motor sailing yachts with enormously powerful engines! However, significant numbers of perfectly "normal" local cruising yachts, most of which are under 40 ft, routinely cruise up and down the St Lawrence each summer. It is possible, it just needs time, patience and careful planning.

In the latter context, irrespective of which direction you are heading it is absolutely essential to have a copy of the official Canadian Hydrographer Tidal Stream Atlas on board before setting out. Anyone trying to rely on the tidal diamonds on the charts, or the tidal flows shown on some electronic charts, will quickly get into trouble (especially if using Navionics charts where the displayed tidal flows for this area are often 180 degrees out!) . Tidal flows in the St Lawrence east of Quebec are far from rectilinear, with a veritable maze of counter eddies along the shore and between the mid stream islands, and extremely strong cross tides setting across banks, spits and other shoal water. 

Once east of the Tadoussac river mouth the tides and currents quickly become weaker and more unpredictable with wind driven currents and other local effects, such as funnelling between islands and freshwater flows from rivers flowing into the St Lawrence, coming in to play. However,  the net effect is a current of around  0.5 to 1 knot flowing in a northeasterly direction, but with occasional opposing  flows of up to 1.5 knots in places.

When heading outbound there are a small handful of tidal gates where one needs to get the timing right,  but other than that you can sit back and enjoy the ride. The biggest logistic problem sailing vessels may face when heading downstream is where to refuel, particularly for vessels with a draught of over 1.5m as many of the harbours dry or are shoal.  In settled weather running dead down wind with a strong current underneath you means that often the relative wind over the deck may often not be sufficient to sail and fuel consumption may be higher than anticipated. 

Although on the face of it the inbound sailor faces an unremitting beat to windward, there are one or 2 tactics which can be employed to aid progress. The first of these is that the heavily wooded and mountainous slopes on either bank results in a significant backing of the wind ( sometimes as much as 30 degrees or more) close to shore compared to mid river. This means that when tacking up river it generally pays to pick one shore or the other and then to short tack in the zone where the wind is backed, rather than putting in long tacks from one side of the river to the other. This tactic has the added benefit that it avoids repeatedly tacking across the separation scheme that runs down the middle of the river.

The second is that with this backing of the wind, in a southwesterly wind the south shore is the convergent shore with the backed wind from the land reinforcing the wind over the sea to produce an acceleration effect. Added to this can be a katabatic wind effect coming down of the mountains of the southern shore. The net result is that in the prevailing southwesterly winds, wind strengths can be significantly stronger on the southern shore than on the north. 

If the wind  is light this acceleration on the southern shore may provide a welcome addition to the gradient wind, giving enough wind to sail at a reasonable speed, whilst in stronger conditions the northern shore may give smoother water and more comfortable sailing conditions.

Finally, even in high summer water temperatures on the St Lawrence are generally low, with as little as 4C in the outer stretches and around 10C closer to Quebec. This makes for perfect sea breeze conditions and on a clear day the sea breeze can reach 20 knots plus. East of Tadoussac the river is wide enough that a sea breeze  forms independently on both shores and in such conditions it pays to hug one shore of the other and benefit from the lif

CAUTION: The information above is selective and reflects conditions at the time of visiting. It is not definitive and may be changed or revised without notice. To the extent permitted by law, the RCC Pilotage Foundation and contributors do not accept any liability for any loss and/or damage howsoever caused that may arise from reliance on information in this Cruising Note and any attached files. The RCC Pilotage Foundation would welcome additional information or corrections to the information in this note. Please click here if you want to provide feedback on this or any other notice.