The production of the Atlas is made possible through the
funding of the National Search and Rescue Secretariat and
the collaboration of Parks Canada and its personnel in
the Québec Service Centre and in the Mingan Field
Unit. The personnel of the Ocean Sciences Direction, Fisheries
and Oceans Canada, were put to contribution in the data
collection and analysis, and in running the numerical model
of circulation from which the surface currents are extracted.
The Canadian Hydrographic Service supported the project
in providing bathymetry and water level data of the region.
The Atlas shows the surface currents in Mingan Archipelago,
where the tides and the freshwater outflow dominate the
circulation. This Atlas presents currents produced
by tides and the river flow from Mingan and La Romaine
during the year, except in the spring freshet period, but
does not account for wind-driven current.
The currents shown in the Atlas have been validated
by the personnel of the Ocean Sciences Direction, Fisheries
and Oceans Canada, Quebec Region at Maurice Lamontagne
Institute in Mont Joli.
|An overview of the
- The Atlas covers, over three geographical
sectors, the Mingan Archipelago, between the island Aux
perroquets and La Grande Pointe.
- For each sector of the Atlas, twelve currents charts
show, over time intervals of one hour, a typical semi-diurnal
- The charts use Mercator projection.
- The Atlas is used in conjunction with the Canadian Tide and
Current Tables (Volume 2), prepared annually by the Canadian
Hydrographic Service or on Internet, in order
to access the corresponding water levels for the region at the port
of Havre Saint-Pierre or Mingan.
- The surface currents are illustrated by arrows (vectors)
grouped in triplets. The first vector of each triplet
illustrates the mean current during the first twenty
minutes, the second vector illustrates the next 20 minutes
and the third vector the last 20 minutes period of the
- The current speed is expressed in knots (1 knot =
1 nautical mile per hour, or 1.852 km/h). Current arrows
represent averages over the top five (5 ) metres of the
- The currents represent the mean tidal conditions met
during the year, except in the spring freshet period.
In period of lower or higher tidal range, the surface
currents will be weaker or stronger, respectively. A
correction factor on the current speed for the tidal
range could not be established due to the limited observations
in the region.
- The Atlas does not account for wind-driven currents.
- Variations in currents over distances less than 200
metres are not accounted for in the Atlas.
- Currents with intensities less than approximately
half a knot may actually be in a direction opposite to
the observed condition.
The tides in the region of the Gulf of St. Lawrence, covered
by this Atlas, are dominated by semi-diurnal (twice daily)
oscillation with an average period of 12 h 25 min. The
tides are mixed, in that there is also a diurnal (daily)
oscillation interacting with the semi-diurnal to produce
semi-diurnal cycles that differ in height and duration
from one cycle to the next. The tides are generated in
the Atlantic Ocean and transit in the Gulf of St. Lawrence
as waves. Tides produce the most significant and forecastable
currents in the St. Lawrence Estuary between Trois-Rivières
and the Saguenay River, and in Mingan Archipelago. As one
exits the immediate vicinity of the Archipelago, surface
currents are dominated by wind-driven and density currents
over the tidal ones and cannot be presented in an Atlas
form. For the surface currents outside this region, we
advise the sailor to use the surface currents forecast
issued daily on Internet at St. Lawrence Global Observatory
web site: SLGO.ca under
the section: Ocean
|How to use the Atlas
|Selecting the appropriate chart
- Click on one of the three sectors to access a specific
- With the confirmation of the selected sector, a
smaller pop-up window allows to input the parameters
: the date and time of the request.
- Modify the parameters if needed, then submit the
request. A chart with the surface currents for the sector,
date and hour requested will be shown.
- To access the chart of the surface currents for
the previous hours, click on the “Previous Hour” button
the number of times needed. Similarly, to access the
chart of surface currents for the next hours, click on
the “Next Hour” button the number of times
needed. One can access those charts for the six hours
before or following the hour indicated on the initial
- The target on the top left of the screen can be used to locate a
position and keep it from one chart to the next. One can
follow the drift of the water on one chart, locate the
end point with the target, and
keep this position to the following chart to continue
the drift according to the new set of currents. Click
once on the target to be able to move
it. Drag it to the location needed. Double click on it
to release it. Click on “Previous Hour” or “Next
reach another chart. If the window size is kept the same,
the target should
stay on the same location. The target will come back to
its initial position upon a new request from the pop-up
|Estimating the current
Current speed is estimated by first comparing the colour
or length of a vector with the speed scale shown on each
chart. Current direction relative to true North is estimated
by measuring the direction of an arrow relative to the
meridian line on each chart.
Each arrow also represents the path of the water mass
or a floating object under the influence of that current.
The distance represented by this path is denoted as the “drift”.
You can estimate the drift by measuring the length of the
arrows on the scale shown, or on the meridian axis, knowing
that one minute of latitude represents one nautical mile.
Each arrow represents the drift of the mean current over
the distance covered by its length within a period of 20
minutes. The arrows are grouped in triplets representing
20 minutes each, thus covering a one hour interval.
|Wind waves and Small Craft Warning
Winds blowing against the current can generate very large
and steep surface waves that are particularly hazardous
to small craft. At times these waves may outlast the wind.
The Atlas of tidal currents is produced by a three-dimensional
hydrodynamic numerical model using a grid with 200 m horizontal
resolution and 12 layers vertically. This model solves
the equations of hydrodynamics using the bottom topography,
the mean runoff from the tributaries, La Romaine and Mingan
River, the tidal predictions and the mean water structure
(layers of different densities). In this model, the water
density is free to evolve with the currents and the turbulent
mixing. The results of the numerical model have been compared
with numerous water level and current measurements acquired
at various positions in July 1997.
Once the comparisons were made and the models adjusted,
a synthesis of the information was required to produce
the Atlas in a limited number of pages. The variability
of the currents was analysed as a function of the variable
length of the semi-diurnal cycle, and changes in water
density. The method that was retained to establish the
Atlas was to reproduce a typical semi-diurnal
cycle. A semi-diurnal tidal period
thus covered by 12 charts. The semi-diurnal cycle has an
average length of 12 h 25 min with a standard deviation
of 25 minutes. The method to decompose the time
intervals is judged as a compromise between the simplicity
and the precision required to produce the Atlas.
During July 1997, surface currents were sampled using drifting
buoys equipped with GPS (Global Positioning System) receivers.
These samples represent approximately 260 drifts of 20
minutes each. Model results were compared with the recorded
drifts. One must note that the charted currents with intensities
less than about half a knot may actually be in a direction
opposite to the observed condition. This method of comparison
is similar to the one used in the case of the Atlas
of tidal currents for the St. Lawrence Estuary, from Cap
Bon Désir to Trois-Rivières.
The comparison between the currents illustrated in the
Atlas and the observed trajectories of surface drifters
shows that the model reproduces well the tidal currents
in the Archipelago, and in particular the timing of the
tidal slack currents, that is when the tides turn from
ebb to flood and conversely. This comparison also showed
the clear influence of the large scale wind-driven currents
of the Strait of Jacques Cartier on the currents in the
Archipelago. This large scale current is generally parallel
to the coastline in one direction or another, depending
on the direction of the wind, and must be added to the
tidal currents. This added contribution is stronger in
sector number 1 (the region to the west of the islands à
than in the two other sectors. Thus, it is advisable to
consult the surface current forecast for the Strait of
Jacques Cartier to account for wind effects and add this
contribution to the tidal currents. This wind-driven surface
current forecast is issued daily on the Web site of the
St. Lawrence Global Observatory (referenced to in the above section
on Tides). This forecast makes use of the wind forecast
for the current day. The influence of the wind-driven large
scale currents in the Strait of Jacques Cartier on the
observed trajectories has also the effect to make impossible
to quantify the mean error on the currents shown in the
Atlas. Prior to this mean error estimate, a quantitative
evaluation of this influence has to be done.
|Sources of errors in the Atlas
|Winds, Storm Surges and Mean Water Level
The influence of the wind is not accounted
for in the Atlas. The intensity, direction, duration and
fetch (distance over which the wind blows) are all important
parameters altering the surface current. It is therefore
difficult to realise a complete prediction, accounting
for the wind-driven currents, that can be used in the Atlas.
The most frequent and average winds produce currents that
are smaller than the tidal ones. However, under conditions
of strong winds the effects may be significant. It is the
user's responsibility to judge how to use the Atlas in such
The effects of storms, occurring in the Gulf of St. Lawrence
or as far as the Atlantic Ocean, can modify the mean sea
level over periods of a few hours to a few days. These
phenomena, called storm surges, can modify the tidal propagation
well as the accuracy of the current estimate.
To sum up, variations in atmospheric pressure as well as
persistent strong winds can change the timing of slack
water, either running late or in advance, and influence
the intensity and direction of the currents shown in this
The variations in the currents at scales
less than 200 metres are not accounted for in the Atlas.
For instance, the current near an obstacle (a rock, a shoal,
a pilar, the shore or a wharf), may be smaller, oriented
differently or stronger in the vicinity of it. It is the
average of the current in the neighbourhood of the obstacle,
within a radius of approximately 200 metres,
that is shown in the Atlas. Each current vector must be
considered as an average over the distance between neighbouring
- Atlas of Tidal Currents, St. Lawrence Estuary
from Cap de Bon Désir to Trois-Rivières.
Minister of Fisheries and Oceans 1997, Catalogue No Fs-72-33/1997,
- Saucier, F.-J., J. Chassé, M. Couture, A. D'Astous,
R. Dorais, D. Lefaivre, and A. Gosselin. The making of
a surface current atlas of the St. Lawrence". Proceedings
of the Conference on Coastal Engineering 1999, Lemnos,
Greece. J. Computational Mechanics, Wessex Institute
of Technology, 87-97, 1999.
- Canadian Tide and Current Tables (Volume 2), Department
of Fisheries and Oceans, Canadian Hydrographic Service,
Annual publication, Ottawa (Ontario).
- Sailing Directions, St. Lawrence River, ATL 110, 2nd
Edition, 2002, Department of Fisheries and Oceans, Canadian
Hydrographic Service, Ottawa (Ontario).
- Forrester, W. D. 1983. Canadian Tidal Manual, Department
of Fisheries and Oceans, Canadian Hydrographic Service,
Ottawa (Ontario), 148 p.
- Dohler, G. 1986. Tides in Canadian Waters, Department
of Fisheries and Oceans, Canadian Hydrographic Service,
Ottawa (Ontario), 18 p.
- Thomson, R. E. A brief description of tides, tidal
currents and waves. Articles selected from a series published
in Pacific Yachting magazine.