Section 5: Impacts on Various Interests

5.1 General

5.1.1 What is the value of high water? Who is impacted?    

High water levels generally benefit municipal water users, shipping, hydropower generation, and recreational boaters, although extremely  high levels and fast currents may be dangerous and unfavorable to these interests too. Ships may be unable to pass under bridges and overhead cables. They negatively impact riparian owners by overtopping fixed docks or flooding buildings close to the water’s edge. In addition, shoreline erosion rates are often greater with higher water levels, and shoreline protection structures can be damaged with extreme high water levels. Wetlands extend further inland with higher water and are inundated to greater depths, which is beneficial from time-to-time. Periodic high water levels are necessary to sustain wetland productivity and health, though extremely high levels can impede such benefits.

5.1.2 What is the value of low water? Who is impacted?    

Low water levels impact municipal users as the water intakes may be above the water surface or affected by warmer and poorer quality water. Low water levels result in less water available for dilution of municipal waste water, or increased cost for treatment. Shipping and boaters are adversely affected by low water as channels are shallower. Ships may need to lighten their load to decrease the draft and recreational boaters may find that docks or launches do not extend sufficiently far. Hydropower generation is affected by lower available head or lower flows.

Low water levels benefit riparian owners whose docks may be short and low or who have buildings close to the water’s edge. Erosion rates generally decrease with lower water levels.

Periodic low summer water levels can also result in positive environmental benefits. Low water levels expose more beach and mudflats. Additionally, wetland vegetation may extend further into the lake and river. Wetland dewatering encourages dormant aquatic seed bed germination, benefiting wetland plant species and biodiversity.

5.1.3 At what Lake Ontario level(s) do adverse impacts from low or high water occur?

No single critical low or high water level has "adverse impacts" in the system. The level of harm experienced by a user of the system greatly depends on the location and the usage and sometimes the time of the year, since stormy weather and high winds may cause crashing waves, damaging shorelines at any high water level. The Board is aware that various sectors in the system may be impacted as levels in Lake Ontario and the St. Lawrence River vary, even within their historic ranges.

5.1.4 What can shoreline communities do, if anything, to assist the Board?   

Shoreline communities can help the Board by communicating with us so that we understand your situation and concerns. More importantly, shoreline communities can help themselves by becoming better informed about coastal hazards and taking action to reduce their risks to such hazards, particularly through incorporating risk-reduction measures in their shoreline revitalization efforts and flood response and recovery plans as well as their long-term planning of coastal zone uses. Shoreline resilience refers to measures that acknowledge the likelihood of extremely high and low water levels from time-to-time.

5.1.5 What have been the impacts of regulation on Lake St. Lawrence compared to preproject conditions?

Prior to the construction of the Moses-Saunders Power Dam, the Long Sault Dam, and the associated control structures, the St. Lawrence River upstream of the project was much narrower, containing fast-moving water and rapids, and was subject to ice jams and associated flooding. Construction of the Seaway and power project stabilized flows and created Lake St. Lawrence, which is the large headwater pond or “forebay” upstream of the Moses-Saunders Power Dam. The wider river flooded some historic villages and farms on both sides of the river. People who lost their homes had to relocate and many homes were moved to new sites upland. The projects also allowed for the growth of recreational boating and associated businesses in this area. Without the project and associated regulation, much of the St. Lawrence River between Ogdensburg and Massena, NY or Prescott and Cornwall, ON would not be navigable for commercial or recreational vessels - except for canoes, kayaks, and rafts.

5.1.6 What water level information is available?

As water levels affect many interests, perhaps one of yours, the Board has many ways to communicate water levels in the system. The “Watershed” tab on our website indicates “Current and Expected Conditions” (levels and flows) throughout the System. Interested parties may use this link to subscribe to announcements of weekly specified flows and levels. The “News and Events” tab on the website lists news releases. The Board posts snapshots of water levels from around the Lake Ontario-St. Lawrence River System regularly on its Facebook page. Other sources of water level information are available directly from the agency involved: NOAA, CHS, Seaway, ORRPB.

The Board urges everyone to be prepared to live within the full range of levels that have occurred in the past and of those that may occur in the future. Based on historical observations and projected future conditions, at a minimum, Lake Ontario water levels are expected to range from a high of 75.92 m (249.1 ft.) to a low of 73.56 m (241.3 ft.) at infrequent intervals. However, it is also recognized that future climate conditions are uncertain, and more extreme water levels may be reached and these extremes may occur more often. Levels on the St. Lawrence River tend to vary more widely than on Lake Ontario. Also, these levels do not include the varying local effects of strong winds and wave action that significantly increase or decrease local water levels on both the lake and river, with temporary changes of over half a meter (two feet) possible in some locations.

5.1.7 What adaptive measures should water users and property owners be taking above and below the Dam to protect against adverse impacts, and will the Board be taking actions to encourage such measures to be implemented? 

The Board always encourages water users and property owners to plan for the full range of water levels that have been experienced historically. Recommending specific adaptive measures is beyond the purview of the Board, but information on this topic is readily available online, or from your local officials and conservation authorities.

5.2 Domestic Water Supplies and Sanitary Uses

5.2.1 How can high or low Lake Ontario water levels affect domestic and sanitary uses?

High water levels can threaten domestic water supplies and sanitary uses. These consist of flooding and backup of sanitary sewer lines, flooding and contamination of freshwater storage wells, and under very high levels or during storm events, possible flooding of freshwater intake pumping facilities.

Very low water level conditions may affect shore wells and municipal and industrial water intakes on the St. Lawrence River and along the entire Lake Ontario shoreline. Such impacts can be avoided through proper design of such facilities with anticipation of the entire range of Lake Ontario levels and flows and those that occur on the St. Lawrence River.

5.3 Commercial Navigation

5.3.1 How can the regulation of Lake Ontario outflows impact commercial navigation?

The construction of the Seaway and power project opened the Great Lakes to ocean-going navigation. Outflows exceeding design limits in the regulation plan can result in severe water velocities and cross-currents in the river, which can occur at key points such as at entrances to navigation locks, causing difficulty in controlling the vessels. In addition, very high flows at the Moses-Saunders Power Dam can reduce the available draft on Lake St. Lawrence, immediately above the dam, due to a drawdown effect at the dam and immediately upstream.  During outflows above about 9,900 m3/s (once considered the limit for commercial navigation) the Seaway Corporations impose certain speed and passing restrictions, and tugs may assist ships into the locks, ships are restricted from passing in critical river reaches and high-powered bow thrusters are required to help steer the ships through the strong currents in the river. Some tug and barge transits and tall ships may be restricted from entering the system.

By contrast, very low outflows can result in extremely low levels in downstream areas of the Seaway and at the Port of Montreal, limiting the ability of some vessels to transit the channels without grounding and/or approach docks and piers. Under certain conditions, shippers may have to lighten their vessels, forcing them to either leave cargo behind or increase the number of transits necessary.

5.3.2 Are navigation interests impacted by Lake Ontario water levels?

Yes. While high water levels on Lake Ontario can produce benefits for navigation, they often mean higher water levels in the St. Lawrence River, which can have negative impacts. Ship passage may be restricted by bridges and cables which do not allow sufficient “air gap” when water levels are high. High river flows can result in transit delays and more fuel being necessary for upbound traffic. Low levels can also be hazardous to navigation and can increase the risk of ship groundings. Shippers may have to lighten their vessels, forcing them to either leave cargo behind or increase the number of transits necessary.

5.3.3 Can the navigation season between Lake Ontario and Montreal close earlier so more water can be let out of Lake Ontario?

The Commission does not specify either the start or the end of the navigation season. This season is jointly determined by the Seaway Corporations. The regulation of outflows in accordance with the Boundary Waters Treaty and the IJC’s Order of Approval, however, does take into account navigation. Under normal circumstances, however, shortening the navigation season a week or two at either end would rarely result in significant opportunities to alter flows substantially, so doing so would not offer measurable benefits with respect to Lake Ontario levels. When criterion H14 is exceeded by high water supplies, the Board has the authority to deviate by releasing outflows to the detriment of shipping, especially if the advantages to shoreline property owners are substantial and evident. 

5.4 Hydropower 

5.4.1 Do hydropower interests benefit from high Lake Ontario outflows?    

For hydropower, more electricity can usually be generated when there is a greater volume of water passing the turbines. However, the consequent lowering of upstream levels decreases the head on the hydropower stations, thereby decreasing the amount of electricity generated for a given volume of water. Finally, extremely high outflows may exceed the capacity of the hydroelectric facilities, resulting in water being diverted around the generating equipment and being lost in terms of power generation. Running the turbines at maximum capacity for an extended period of time also reduces the opportunities for maintenance of the power units while increasing the likelihood of them requiring it.

5.4.2 When the Board increases outflows from Lake Ontario is the water spilt or is it still used to generate electricity?

The generating capacity of the Moses-Saunders Power Dam is around 9,300 m3/s when all units are available under typical conditions. Since the average outflow is 7,360 m3/s, the water is not typically spilt when outflows are increased, especially during dry conditions when the outflows are increased to augment low levels downstream. The water passes through the generators. Therefore the spillway at Long Sault dam is rarely in use, but was employed for limited periods in 1998, 2017 and 2019, for example. When spilling is needed, normally just several gates at the Long Sault dam are employed, and it is very unlikely that all the spill gates would ever be needed.

5.5 Environment 

5.5.1 What authority does the ILO-SLRB have to consider the environment when setting Lake Ontario outflows?

The 1956 Order did not include consideration of the environment. Thus, the Board had no direct authority to consider it in its deliberations under that Order, determining that little benefits to the environment on Lake Ontario were possible but some limited benefits could be achieved in specific instances in the St. Lawrence River. Since the 2016 Order, Plan 2014 hopes to allow more divergent water levels in times of normal water supplies than the previous plan. The more-frequent fluctuations of water levels on Lake Ontario are expected to result in benefits to wetlands and ecological factors over the long-term.

Note that, during extremely wet or dry periods, the intent of Plan 2014 shifts to returning balance to the system as quickly as possible, and environmental benefits are sidelined until conditions return to normal.

5.5.2 Had regulation had an adverse impact on wetlands due to the reduction of the occurrence of extreme high and low water levels on Lake Ontario from 1960 to 2016?   

Yes, the Lake Ontario-St. Lawrence River study concluded that regulation had a negative impact on the wetlands and wetland-supported habitats of the Lake Ontario-St. Lawrence River Basin. Under the previous 1956 Order of Approval, the regulations of Plan 1958-D strived to keep Lake Ontario water levels within a 1.2-metre (four-foot) range, which didn’t allow for the variable water levels a diverse wetland requires. While each wetland is unique, narrowing the range of water level fluctuations generally results in less wetland acreage and less diversity within the wetland plant communities. The Commission's Levels Reference Study Board concluded in 1993 that the reduction in the range of water level fluctuations resulting from regulation adversely affected the extent and diversity of Lake Ontario's wetlands. It also concluded that altering natural water level conditions on Lake Ontario resulted in the appearance of undesirable plant species in its wetland habitats and limited the capability of wetlands to support the overwintering water-dependent fish and wildlife populations. Further, there was also concern regarding the environmental impact of outflow regulation on wetland and fishery habitats in downstream areas of the river. More definitive data regarding all these impacts was provided by studies conducted by the Lake Ontario-St. Lawrence River Study for the IJC from 2000 to 2006.

5.5.3 How has regulation impacted the environment upstream of the project at Massena and Cornwall?

The construction of the dams at Massena and Cornwall created Lake St. Lawrence immediately upstream, thus changing the nature of that environment from a river to a lake environment. That change has brought about physical, chemical, and biological modifications. There are also concerns regarding impacts from the use of the shipping channel (e.g., enhanced shoreline erosion, invasive species, etc.).

5.5.4 How has regulation impacted the environment downstream of the project?    

Regulation has reduced the number and, perhaps more critically, the timing and duration of extreme high and low levels on the river downstream. More definitive data regarding the impacts on wetlands, water fowl, wildlife and fishery was provided by studies conducted by the Lake Ontario-St. Lawrence River Study for the IJC from 2000 to 2006 (

5.6 Riparian and Shoreline Property Owners 

5.6.1 What factors affect shoreline flooding and erosion?    

The major factors affecting long-term erosion rates include shoreline materials and structure, long-term patterns of sediment transport along the shore, and water levels. These factors have caused erosion since the last ice age and will continue to cause erosion in the future.

Strong winds are responsible for significant erosion over short time periods. When water levels are high, wind-driven waves can trigger significant short-term erosion events that would otherwise occur later or more gradually. They may also direct wave energy against existing erosion protection structures, sometimes resulting in damage. With extremely high water levels, waves may also directly impact and damage dwellings and other structures near the shoreline. High water levels also result in localized flooding of roads and other public facilities and may result in damage to private property as well as public water and sewage facilities. The Lake Ontario-St. Lawrence River Study found that damages to shoreline property and shoreline protection structures increases with increasing mean water levels on Lake Ontario. While long-term recession rates appear to be independent of water level fluctuations for some Great Lakes shoreline areas, water level fluctuations do play a role in determining long-term recession in others. The water level fluctuations may also influence beneficial shoreline sand re-deposition and sand dune replenishment. More definitive data regarding all these impacts was provided by studies conducted by the Lake Ontario-St. Lawrence River Study for the IJC from 2000 to 2006.

5.6.2 What have been the benefits to St. Lawrence River riparian property owners as a result of Lake Ontario regulation?   

Timely management of the Lake Ontario outflows has consistently prevented extremely high water levels and flooding in the Montreal region. Regulation of Lake Ontario outflows has also greatly reduced the incidence of ice jams in the river, both upstream and downstream of the Cornwall/Massena area, thus reducing the flooding and shoreline damage often associated with these events. Finally, the construction of the St. Lawrence Seaway and power project created conditions enabling the establishment and growth of recreational boating on many areas of the St. Lawrence River. This occurred through the elimination of rapids, the creation of Lake St. Lawrence immediately above the dam, and the stabilization of river flows and water levels. The shores of both Lake St. Lawrence and Lake St. Francis are largely protected from flooding owing to the presence of large dams and other control structures at their upstream and downstream ends.

5.6.3 Are there ways to address flooding and erosion problems other than through regulating water levels?

While shoreline protection may be effective in some instances, the best way to address flooding and erosion problems is through effective shoreline management. This is primarily a local, state, or provincial government function. The New York State Department of Environmental Conservation regulations pursuant to the NY State Coastal Erosion Hazard Act implement several actions to reduce flooding and erosion losses along the Lake Ontario shoreline. The Provinces of Ontario and Quebec address flooding and erosion problems with flood-plain management measures that provide easements and restricted building zones along the shore.

Actions by the NYSDEC and the Provinces of Ontario and Quebec that have been found to be effective include:

  • For new structures, flood and erosion setbacks that consider the long-term recession rates;
  • Prohibitions on removing or damaging natural shoreline protective features such as bluffs and dunes;
  • Prohibitions of any construction or structures within flood hazard areas;
  • Relocation of structures at risk;
  • Real estate disclosure requirements;
  • Acquisition of high-risk undeveloped land;
  • Limiting construction in flood plains; and
  • Purchase of flood easements within flood plains.

5.6.4 What can I, as a landowner, do to protect myself from high water damages to my property? What can the Board do to help me?

As a landowner, it is important to build with the knowledge that water levels fluctuate, and to know the extremes possible at your location. The Board urges everyone to be prepared to live within the full range of levels that have occurred. The Board strives to maintain the range of monthly mean levels of Lake Ontario specified in the Order of Approval. However, the control of outflows is an imperfect tool when faced with extremely wet or dry water supplies. Know the extent to where high water and winds act and build accordingly. Structures, such as breakwaters and heavy armor stones may protect against strong wind action when built correctly. Residences should be built with sufficient setback to avoid flood and erosion damages in the future. The Board announces weekly levels and flows on its Facebook page and via e-mail; you may sign up to receive the e-mail bulletins. The publication at contains some guidelines on resilient shorelines.

5.7 Recreational Boating

5.7.1 Other than water level regulation, are there any actions that could benefit recreational boaters?

Recreational boaters have problems navigating at different points on the lake and on the river, even when levels and flows are within the ranges in the Order. For example, a private dock or a marina built during high water levels may not be fully usable when water levels fall. Therefore, one action to alleviate this is to site, design, and maintain new and existing recreational boating facilities - taking into account the full range of water levels and flows, at minimum as outlined in the Order and as have occurred in the System. This may necessitate the use of longer launch ramps, floating (rather than fixed) docking systems, and the financial commitment and permits to perform periodic maintenance dredging as needed to accommodate the planned use. Also, it must be recognized that some areas are shallow and cannot accommodate dockage for large recreational vessels. In addition, boaters should pay particular attention to navigation charts during low water periods, even in waters with which they have familiarity.