The Pre-project plan with Ice Limits
Background (Caldwell and Fay 2002)
Prior to regulation, Lake Ontario outflows were limited by the hydraulic capacity of the St.
Lawrence River channel. Rock sills at the head of the Galop Rapids (in the vicinity of Galop and
Adam Islands) formed the natural constraint. Originally, these two islands divided the rapids into
three channels: the Canadian Galop Rapids, the Gut, and the American Galop Rapids. This
section was modified to facilitate navigation in the late 19th and early 20th centuries by dredging
(of the Canadian Galop Rapids) and construction of a submerged weir (Gut Dam). This weir was
removed by January of 1953. In its Orders of Approval for the Regulation of Lake Ontario, the
International Joint Commission (IJC) defined the pre-project outlet conditions as those existing
between 1953 and 1955, after the removal of the Gut Dam, but prior to the beginning of the St.
Lawrence hydropower project. This state is hydraulically similar to the natural state of the
channel prior to 1900 (ILOBOE, 1958).
In its natural state, the river fell approximately 1.5 m in 1.6 km within the Galop Rapids
(CCGLBHHD, 1958). From Lake Ontario to the head of the Galop Rapids, the fall was 0.6 m over
a distance of 112 km. Below the rapids, the river fell 4 m over the next 16 km to the head of
Rapide Plat (rapids that formerly existed adjacent to Ogden Island). The channel constriction at
the Galop Rapids was sufficient to create a backwater effect, and flow at these rapids reached
speeds in excess of what is defined (hydraulically) as “critical velocity”. In other words, water
levels upstream of the rapids were independent of levels and flows below the rapids. As such, a
stage-discharge relationship could be defined that was dependent only upon upstream level
conditions.
Such a stage-discharge relationship to estimate pre-project outflows was developed in the 1950s
for the International St. Lawrence River Board of Control (ISLRBC) and was reviewed and redeveloped by Caldwell and Fay in 2002. Determination of pre-project flows in the winter must
take into consideration flow retardation due to ice conditions. Ice retards (i.e., reduces) the
outflow in the St. Lawrence River and generally results in a temporary increase in water levels
upstream of the ice formation. The relationship includes terms to account for ice retardation
effects upstream of the Galop Rapids. As the land and channel bottom at Galop was slowly
rising relative to the land and lake bottom of Lake Ontario, under natural unregulated conditions
the water level of Lake Ontario was gradually rising over time (all else being equal). A term is
included in the pre-project stage-discharge equation to account for this gradual rise in the
elevation at Galop compared to Lake Ontario level.
Since pre-project outflows may be estimated using a stage-discharge relationship, the effects on
lake levels are obtainable, and thereby pre-project levels may be determined. The Lake Ontario
outflow and level regime that would exist under unregulated conditions can be simulated by the
use of this pre-project stage-discharge equation.
The Pre-project Regulation Plan
The release for this plan is calculated according to the pre-project Lake Ontario stage-discharge
relationship developed by Caldwell and Fay (2002). The term to account for differential crustal
movement in the equation has been fixed to simulate conditions as they would be in the year
2010. The historic pre-project winter ice retardation effects are included in the determination of
the pre-project flow.
As with all regulation plans, this Pre-project regulation plan would function with the modern
channel and structures in place in the international section of the river. Additional rules, similar
to those used in Plan 1958DD and Plan 1998, have been incorporated into this plan to limit the
maximum release in the winter in order to form and maintain a stable ice cover on the river and
prevent ice jams. These added rules cause the results of this plan to be different than those that
would occur under a purely unregulated system.
This plan assumes a perfect forecast of the coming quarter-month’s supply to Lake Ontario, but
does not use any forecast of inflows to Lake St Louis.
References:
Caldwell, R. and Fay, D., 2002. Lake Ontario Pre-project Outlet Hydraulic Relationship
Final Report. Hydrology & Hydraulics TWG, IJC Lake Ontario - St. Lawrence River Study.
Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data, December 1958.
Lake Ontario Outflows 1860-1954.
International Lake Ontario Board of Engineers, October 1958. Effects on Lake Ontario Water
Levels of the Gut Dam and Channel Changes in the Galop Rapids Reach of the St. Lawrence
River.