Lake Ontario does not experience the daily rise and fall of water levels associated with true oceanic tides. A tide is a phenomenon caused by the gravitational forces exerted by the Moon and the Sun, which create a bulge of water on opposite sides of Earth. While this gravitational pull affects all water bodies, including Lake Ontario, the effect on large, enclosed lakes is too minor to be noticeable to a casual observer. The water level fluctuations that people do observe on the lake are instead the result of atmospheric conditions and the lake’s position in the larger Great Lakes system.
Why Great Lakes Do Not Experience Ocean Tides
The absence of significant tides in Lake Ontario is primarily due to the lake’s small basin size and its enclosed nature. True oceanic tides require vast, interconnected bodies of water for the gravitational differential to become a significant force. The gravitational pull acts differently across the massive scale of an ocean, creating a substantial difference in water level between points. Lake Ontario’s maximum length is about 193 miles, which is not enough distance for the gravitational pull to create a large difference in water height. Although a micro-tide does exist, its maximum rise is less than two inches, and this minuscule gravitational effect is completely overshadowed by other, more powerful meteorological forces acting on the lake.
The Phenomenon of Seiches and Wind Setup
The noticeable and sometimes rapid changes in Lake Ontario’s water level are caused by weather-driven events known as seiches and wind setup. Wind setup, also referred to as a storm surge, occurs when strong, sustained winds blow across the lake’s surface, pushing the water toward the downwind shore. This action causes the water level to rise significantly at one end while simultaneously dropping it at the opposite shore. Differences of several feet in water level can occur between the eastern and western ends of Lake Ontario due to a sustained wind setup.
When the wind stops or a rapid change in atmospheric pressure occurs, the piled-up water begins to oscillate back and forth across the lake basin. This sloshing motion is called a seiche, which is a standing wave in the enclosed body of water. A seiche can be compared to the motion of water sloshing back and forth in a bathtub, and it continues until the water level reaches equilibrium. These oscillations can lead to extreme fluctuations, sometimes up to 16 feet between the lake’s ends, and can continue for several hours. This back-and-forth movement is what observers mistake for a tide because it causes a temporary, noticeable rise and fall of the water line.
Lake Ontario’s Role in the Great Lakes System
Beyond short-term weather effects, Lake Ontario’s water levels are influenced by its position as the last in the Great Lakes chain. The lake receives its primary inflow from the Niagara River, which drains the water from the upper Great Lakes, including Lake Erie. Long-term, gradual level changes are primarily driven by seasonal factors like snowmelt and precipitation over the entire Great Lakes basin.
The water level is also heavily managed by human-made infrastructure to control its outflow into the St. Lawrence River. The Moses-Saunders Power Dam and the Long Sault control dam regulate this outflow, which is administered by the International St. Lawrence River Board of Control. This regulation is designed to balance the needs of hydroelectric power generation, navigation, and shoreline property protection.
While regulation helps to mitigate the severity of extreme high and low water levels, it cannot prevent naturally driven extremes caused by unusually high precipitation or snowmelt. These management and hydrological factors, combined with the seasonal cycles, are the primary drivers of long-term water level trends. These gradual, systemic changes contrast with the rapid, temporary fluctuations caused by seiches and the negligible effect of astronomical tides.