Water levels fluctuate on an annual cycle, rising in the spring and summer due primarily to snowmelt runoff and low evaporation rates, and declining in the fall due to high evaporation rates from the temperature difference between the air (cold) and water (still warm from summer months). On average, the highest lake levels on Lake Ontario are observed between May and July.

Figure 1: Monthly average Lake Ontario water levels from 1972-2021 (blue) with record highs and lows (black).

Water levels in Lake Ontario are regulated through international agreements. These regulations generally lead to less variability in Lake Ontario water levels than the other lakes. Recent years have seen some of the most rapid fluctuations in the recorded history of Great Lakes water levels. Lake Ontario had a slight decline in water levels beginning in 1998, but did not remain below average in the decade that followed, as the upper Great Lakes did. During this time, the lakes were characterized by an approximately 17 year span of warmer temperatures, low ice coverage, increased evaporation rates, and decreased runoff. An increase in  water levels on the upper Great Lakes began in 2014, a year that coincided with a cold air outbreak, low temperatures, extensive ice cover, and high precipitation rates. These conditions continued through the end of the decade, leading to an abundance of water in the entire Great Lakes system, including Lake Ontario.  As a result, Lake Ontario experienced record high water levels in 2017 and 2019.

Figure 3: A closer look at monthly (blue) and long term average (red) Lake Ontario water levels from 2012 to 2021, when water levels rose to above average for much of the decade, before declining again. [Click image to enlarge]

Over-lake precipitation totals are expected to increase, on average, due in part to the ability of warmer air to hold more water vapor (moisture) and, hence, increase precipitation. In the 2010s, Lake Ontario experienced several years of above-average over-lake precipitation totals. This was the wettest decade on record for the Great Lakes basin, collectively.

Figure 4: Annual (blue) and long term average (red) precipitation totals over Lake Ontario from 1972 to 2020.

Lake Ontario summer lake surface temperatures increased by an average of 1.6°C (2.9°F) between 1968 and 2002 (Dobiesz and Lester 2009). This can be partially attributed to earlier spring ice melt which allows for longer periods of lake stratification and more exposure to solar radiation (McCormick and Fahnenstiel 1999). Publicly available lake-wide water temperature data only dates back to 1995, so figure 5 does not capture the long-term or seasonal trends evident from more robust datasets and statistical analysis used in aforementioned studies. It does, however, capture the shift to higher surface temperatures around 1998, that occurred basin-wide.

Figure 5: Annual (blue) and long term average (red) Lake Ontario surface water temperatures from 1995 through 2021.

Lake Ontario had less ice cover on average during the last 20-30 years compared to earlier years, prior to the 1990s (Mason et al. 2016, Van Cleave et al. 2014). However, there remains strong year-to-year variability, meaning that years with very little ice and years with a lot of ice are still possible. In a warming world, there is less potential for large amounts of ice cover, but there are many forces at play (such as cold arctic air blasts) that can still usher in winters of extreme cold, potentially leading to unprecedented high seasonal mean ice cover (e.g., 2014).

Figure 6: Annual average (blue) and long term average (red) ice cover for Lake Ontario from the 1973 ice season to the 2021 ice season. An ice season runs from November 10 the previous year to June 5th of the current year.

These webpages were developed by the GLISA team with funding from National Atmospheric and Atmospheric Administration’s (NOAA) Great Lakes Regional Collaboration Team and Cooperative Institute for Great Lakes Research (CIGLR).

More figures and analysis are available on GLISA’s Lake Ontario Climatology page.