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Water Levels in The Great Lakes

Oct 1, 2017

Author: Fin

Fintan Hartnett is the Principal and Chief Instructor at Topmast Maritime Training and an RYA Yachtmaster Instructor as well as Sail Canada and ASA instructor.

It’s been a fascinating season sailing on the Great Lakes, not least of all for the unusually high water levels experienced this year (2017).  While most RYA students are familiar with the subtleties of tidal heights and secondary port calculations from their theory classes, students here in Kingston have had to contend with the lesser-known vagaries of rising and lowering water levels on Lake Ontario.

When Topmast ran its first course of the year in May there was extensive flooding at Collin’s Bay Marina, our home port, to the extent instructor and students had to wear rubber boots to get to the dock and board the boat.

collins bay marina flooded

Because water levels were so high many marinas and yacht clubs in the area were struggling to accommodate visiting boats.  Pump-outs were impossible for weeks with flooded facilities.  Beaches simply disappeared and impassable shoals were suddenly navigable.  With water levels well over 5ft (1.5m) above chart datum at one point, huge portions of the chart opened up to exploration.  So too did new underwater dangers as the lake hid normally visible rocks and outcrops.  Deadheads were everywhere.

The Canadian Hydrographic Service has adopted the International Great Lakes Datum of 1985 (IGLD85) as its base measurement for depth in the Great Lakes.  Whereas in the United Kingdom and elsewhere we might find chart datum reduced to the Lowest Astronomical Tide (LAT) or the lowest the water has ever been in that area, the Great Lakes suffer from some behavioral issues unrelated to tide such as: precipitation, evaporation, runoff, groundwater, ice retardation, aquatic growth, meteorological disturbances, tides, crustal movements and meteorological disturbances.  Rain, or the lack of rain, has the biggest impact.

Additionally, water levels can be controlled by man (sorry for you King Canute!) and in 2017, because of extensive flooding in Quebec, the powers that be, The International Joint Commission (IJC), had to be very judicious in releasing water into the St Lawrence for risk of devastating La Belle Province.

For those unfamiliar with the geography of North America, the Great Lakes begin at Lake Superior and drain towards the Atlantic.  This amounts to a vertical drop of almost 200m over 3500km.  Due to the size of the Great Lakes this means that a tremendous amount of water (+/- 2000 cubic meters per second) is finding it’s way from Lake Superior through the Great Lakes to what can only be described as a narrow funnel in relative terms: The St Lawrence seaway.

Given the sheer amount of water finding it’s way into Lake Ontario, the IJC was faced with very difficult decisions regarding not only flooding, but the ability of shipping to navigate safely through the St Lawrence seaway.  The economic impact of reducing the shipping has strong economic implications, bearing in mind how important the shipping is to the North American supply chain.

The magic number for chart datum in Lake Ontario is 74m which may beg the question: why?  Somewhere between Montreal and Quebec City chart datum is zero, Lake Ontario sits 74m above that position.  The chart above represents a datum at 74m from which the hydrographic office feels it can safely depict depth on charts of Lake Ontario.  It actually represents the highest lowest low water ever recorded.  Above that line is a curve which represents the average depth of water over the same period and above that the highest recorded lake levels as of 1986.  Below is 2017:

Apart from the record setting water levels there has also been a record amount of water released from the Great lakes as we enter Autumn.  The curve above is steep from August through to October as Lake Ontario is reduced to normal levels for this time of year.  It has affected some of our course structure with some places no longer safe to transit.  Not to mention that moving east beyond Gananoque the water is moving very fast indeed.  Getting back to Kingston can involve pushing against more than 3 knots of current at least.  More importantly, the dock at the Wolfe Island Grill is no longer accessible to vessels with a draft in excess of 5ft (1.5m), luckily we can get the powerboat in there.

For Topmast students the pressure is on early in the course to come to terms with daily lake levels.  It’s important if they want to navigate safely.  Where can they look?  A good starting point is the Government of Canada Fisheries and Oceans website (which can be found on our resource page).

 Lake levels – something to think about.

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