A Product of the Great Lakes Water Quality Agreement

Located along the U.S. Canadian border, Lake Superior is the northernmost of the five Great Lakes, connecting to Lake Huron through the St. Marys River. Lake Superior is the largest lake in the system by volume and surface area, as well as the deepest of the Great Lakes, based on its bathymetry. Inflows to Lake Superior include numerous rivers and the Long Lac and Ogoki diversions, completed in 1941 and 1943, respectively. Both divert water from the Albany River systems in Northern Ontario into Lake Superior for hydropower and logging purposes.

About the Data

Historical observations for each retrospective summary were retrieved from the NOAA-Great Lakes Environmental Research Laboratory (GLERL) Great Lakes Monthly Hydrologic Data, Great Lakes Ice Cover Database, and Great Lakes CoastWatch Surface Environmental Analysis, to provide a snapshot of lake level, precipitation, water temperature, and ice trends in the last 50 years.  The 50-year time period of reference is 1972-2021, with the exception of variables that do not have records that far back (ice cover records begin in 1973, water temperature records begin in 1995).

Lake Levels

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 Superior are observed between July and October.

Figure 1: Monthly average Lake Superior water levels from 1972-2021 (blue) with record highs and lows (black).
Table 1: Average, record maximum, and record minimum monthly water levels (in meters) for Lake Superior from 1972 to 2021.

Recent years have seen some of the most rapid fluctuations in the recorded history of Great Lakes water levels. Lake Superior had a sharp decline in water levels beginning in 1998. Following this decline, the lakes were characterized by an approximately 17 year span of warmer temperatures, low ice coverage, increased evaporation rates, and decreased runoff. A rapid increase in Lake Superior water levels 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 to reach new monthly record highs in 2019. Following basin-wide dry conditions in early 2021, water levels began declining on Lake Superior, returning to average by the end of the year.

Figure 2: Monthly (blue) and long term average (red) Lake Superior water levels from 1918 to 2021. [Click image to enlarge]

Figure 3: A closer look at monthly (blue) and long term average (red) Lake Superior 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 Superior experienced higher over-lake precipitation totals than in previous decades. 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 Superior from 1972 to 2020.
Average (mm)Average (in)Change (mm)Change (in)Change (%)
Table 2: Annual and seasonal averages of overlake precipitation totals on Lake Superior from 1972-2020 and the change in precipitation during the same time period.

Surface Water Temperatures

Lake Superior is one of the fastest-warming lakes in the world (O’Reilly et al 2015). 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). Summer lake surface temperatures increased by an average of 2.5°C (4.5°F) between 1979 and 2006, faster than the surrounding air temperature and any other Great Lake (Austin and Coleman 2007, Zhong et al. 2016). 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 Superior surface water temperatures from 1995 through 2021.

Average (°C)Average (°F)
Table 3: Annual and seasonal averages of surface water temperatures in Lake Superior from 1995-2021. 

Ice Cover

Lake Superior 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 Superior 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.

Note on Overlake Data

Advancement in how measurements are taken (e.g., technical advancements in satellite data and quality control standards) decreases the uncertainty of historical data (Gibson et al. 2019). However, there are particular lake variables that have high uncertainties because of less reliable measurement methods (Fry et al. 2022). Variables like overlake precipitation have higher uncertainty than land-based observations because lake observations are sparse. These observations are then interpolated over the lake, without any consideration for the stability impacts of the lakes on the atmosphere (Holman et al. 2012). Data discontinuities at the US-Canadian border also present challenges (Gronewold et al. 2018). These factors complicate the estimation of overlake observations.

The Great Lakes Water Quality Agreement (Agreement) is a commitment between the governments of the United States and Canada. First signed in 1972 and most recently amended in 2012, the two countries have coordinated to advance protection and restoration of the Great Lakes for 50 years. Promoting research and advancing the understanding of and communicating about climate change impacts was added to the Agreement with the 2012 amendments as Annex 9: Climate Change Impacts. These retrospective summaries, along with their prospective counterparts, were developed to mark the 50th anniversary of the signing of the Agreement in 1972 and provide an overview of past climate and lake trends for the Great Lakes and surrounding basins. These reports were created through Annex 9: Climate Change Impacts to serve the work being done on the other annexes of the Agreement, in particular the Lakewide Action and Management Plans, and natural resources managers and decision makers across the Great Lakes region.

Great Lakes Retrospectives

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 Superior Climatology page.