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.

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An overview of the Atlas

  • 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 tidal cycle.
  • 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 hour.
  • 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 water depth.
  • 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.
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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: under the section: Ocean Forecast.

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How to use the Atlas
Selecting the appropriate chart

  1. Click on one of the three sectors to access a specific one.

  2. With the confirmation of the selected sector, a smaller pop-up window allows to input the parameters : the date and time of the request.

  3. 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.

  4. 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 window request.

  5. 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 Hour” to 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 window.
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.

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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.

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Numerical modelling

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 is 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 de 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 Bouleaux) 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.

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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 conditions.

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 speed as 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 Atlas.

Small-Scale Effects

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 vectors.

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  • 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, Ottawa.
  • 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.
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