Sustained Assessment of the Great Lakes

Lake Superior Overview

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 and is surrounded by several wildlife areas and national parks. Lake Superior is also the deepest of the Great Lakes, based on its bathymetry, and includes inflows from the Long Lac and Ogoki water diversions.

Surface AreaVolumeAverage DepthMax DepthShoreline
82,097 sq. km12,232 cu. km149 m406 m4393 km
(31,700 sq. mi)(2,935 cu. mi)(489 ft)(1,333 ft)(2,730 mi)

Lake Superior basin annotated with major inflows and outflows.

Lake Levels

Variability of water levels in Lake Superior is observed on many different time spans, including seasonal, monthly, annual, and decadal (Figure 1).  The Long Lac diversion was completed in 1941 and the Ogoki diversion was completed in 1943. Both divert water from the Albany River systems in Northern Ontario into Lake Superior for hydropower and logging purposes. Both diversions increased the water supply into the Great Lakes basin and permanently increased the water level of Lake Superior by 3 inches (USACE 2009).

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. Though this coincided with the 1998 El Niño, a causal relationship is not established. 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 record highs on several lakes in 2019. 

Figure 1: Water levels on Lake Superior from 1918 to 2020 (blue), with the long-term average water level (red). Two diversions into the basin (the Long Lac Diversion of 1941 and the Ogoki Diversion of 1943) are marked on the figure.

Lake levels oscillate on annual cycles and multi-year periods of high and lows (Figure 2). The periods of sustained highs and lows are variable in length. The Great Lakes, including Lake Superior, have recently experienced a period of rising water levels, which follows a more than ten-year span of low levels. The past decade was the wettest on record and this record precipitation is the primary contributor to the recent rise in lake levels.

Figure 2: Monthly (blue) and annual (pink) average lake levels for Lake Superior with annotations denoting annual cycles, highs, lows, and the recent rise.

The variability of Lake Superior water levels is also observed on a decadal time scale (Figure 3). The current decade (2010s) of higher water levels occurred after a period of low lake levels (2000s). 

Figure 3: Decadal averaged water levels in Lake Superior (colored horizontal lines) with annual time series (black).

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 are observed between July and October (Figure 4). 

Figure 4: Monthly average water levels from 1918 to 2020 for Lake Superior (blue line) with the record highs and lows (black lines).

The record lows on Lake Superior were set in the late 1920s, with the exception of August and September records that were broken in 2007 (Table 1).  The record highs on Lake Superior were previously  set in the 1980s, until 2019, when water level records were broken for the months of May to September. 


Table 1: Mean, maximum, and minimum water levels in Lake Superior (Source: Army Corps of Engineers)

Net Basin Supply: 

Water levels for an individual lake are driven by the net basin supply (NBS), which depends on the sum of over-lake precipitation and basin-runoff minus over-lake evaporation, as well as how much water flows from one lake to the next.  Human-caused diversions, into and out of the lake, are also part of the NBS, but are much smaller than precipitation, runoff, and evaporation (see Lake Levels Overview page for more information).

Over-lake precipitation varies seasonally, with higher totals occurring in the late spring, summer, and early fall months (Figure 5). This seasonality aligns with the seasonal variability of lake levels, with the highest levels occurring at the same time as the greatest total precipitation. Precipitation totals vary annually as well as seasonally, and do not always follow the same pattern year to year.

Figure 5: Monthly averaged over-lake precipitation for Lake Superior from 1940 to 2019

Runoff rates follow the same general monthly pattern, but can vary in magnitude from year to year (Figure 6).  Peak runoff occurs in late spring and early summer, primarily due to snowpack melt.  Runoff amounts are also influenced by impervious surfaces such as concrete, that prevent water from entering the soil below, and agricultural practices within the basin.

Figure 6: Monthly averaged runoff  for Lake Superior from 1918 to 2019

Evaporation rates follow the same general monthly pattern from year to year. Maximum evaporation occurs in the late fall and winter months with minimum evaporation occurring in the late spring and early summer months (Figure 7).  Because water has a higher heat capacity than air, the lake stays warmer in the fall as air temperature falls. This leads to increased evaporation due to the greater difference in temperature between the air and water. Evaporation is also closely related to high and low ice years. When evaporation rates are high in the fall, signifying more heat loss, it creates conditions for a year with high ice cover. Cold lake surface temperatures and high ice cover then reduce evaporation. The same is true for lower evaporation rates in the fall, signifying less heat loss, and low ice cover years. These conditions can then lead to increased evaporation in the following months (see Ice Cover Overview page for more information).

Figure 7: Monthly averaged over-lake evaporation for Lake Superior from 1950 to 2019

Precipitation, evaporation, and runoff are combined in total net basin supply for Lake Superior (Figure 8). 

Figure 8: Monthly averaged Net Basin Supply for Lake Superior from 1950 to 2015

The variability of over-lake precipitation, evaporation, runoff, and total net basin supply  can also be observed on a decadal time scale (Figures 9, 10, and 11). The most recent decade (2010s) had the most precipitation on record, which contributed to the increase in water levels that was seen throughout the Great Lakes, including Lake Superior. High runoff totals were also observed, primarily due to the high precipitation totals that have been recorded in the 2010s. NBS is the combination of precipitation, runoff, and evaporation, modeled by the equation: NBS = P + R – E  (Figure 12).

Figure 9: Annual Lake Superior over-lake precipitation totals (black line) with decadal averaged over-lake precipitation totals (colored horizontal lines).

Figure 10: Annual Lake Superior runoff (black line) with decadal averaged runoff (colored horizontal lines).

Figure 11: Annual Lake Superior over-lake evaporation (black line) with decadal averaged over-lake evaporation (colored horizontal lines).

Figure 12: Annual net basin supply (solid black line) with decadally averaged net basin supply (colored lines) for Lake Superior.

Lake Ice

There are multiple factors that lead to the differing levels of ice cover between the different lakes including depth, surface area, and latitude. Lake Superior is the northernmost of all the Great Lakes, exposing it to colder temperatures. However, it is also very deep, which allows a higher capacity for heat storage (Figure 13). The large surface area of the lake allows for loss of this stored heat, and therefore more favorable conditions for the formation of ice.

Figure 13: Bathymetry (depth) of Lake Superior.

During the winter months, Lake Superior experiences varying levels of ice coverage. On average, higher ice coverage is observed closer to the coasts and channels due to more shallow water and protection from winds and currents (Figure 14). Below average ice cover was observed during most years in the past decade for most of the lake surface area. Though there were individual years with high ice cover, such as 2014 and 2019, the overall decadal average was low.

Figure 14: Average ice cover in Lake Superior from 2010-2019.

When considering annual maximum or average ice cover in the Great Lakes, it is common to treat each “ice year” as the period between December (of the previous year) and May (of the current year), as this is when freezing events occur in the region. Figure 15 demonstrates this, with peak ice cover between February and March almost no ice cover in the beginning of December and the middle of May.

Figure 15: Decadal averages of daily ice coverage on Lake Superior. Data starts in 1973.

There is variability in Lake Superior ice cover throughout the ice year as well as between different years. Natural modes of climate variability can influence ice coverage. Years with strong El Niño Southern Oscillation (ENSO) events contribute to lower maximum ice coverage, while years with polar vortex intrusions generally lead to higher maximum ice coverage (Figure 16). 

Figure 16: Annual maximum ice coverage of Lake Superior from 1973 to 2021, with color bars.

Lake Surface Temperature

Surface temperatures of Lake Superior follow a monthly pattern with the warmest temperatures occurring in late summer and early fall and the coolest temperatures occurring in late winter and early spring (Figure 17). Overall, the surface temperature of Lake Superior has increased. Between 1979 and 2006, an increase of 4.5° fahrenheit was observed in summer surface temperature. This increase is twice that of the air temperature in the same period. 1

Figure 17: Monthly averaged surface temperature of Lake Superior from 1995-2020 (black line), with monthly average surface temperatures for individual years (grey lines).

Basin Air Temperature

Despite year to year variability, an overall warming trend in air temperature has been observed in the Lake Superior basin from 1948 to 2014 (Figure 18).

Figure 18: Air temperature in the Lake Superior basin from 1948-2010 (black line), with a trend line (green dashed line).

Despite an overall warming trend, very warm or very cold years still occur due to annual variability (Figure 19). There are extreme high and low average temperatures throughout the period of record, but recent average temperatures are generally higher than in the past. 

Figure 19: Ten year running average (blue line), with annual averages (black circles) of air temperature in the Lake Superior basin from 1948-2010.


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  1. Austin, J.A., and S.M. Colman, 2007: Lake Superior summer water temperature are increasing more rapidly than regional air temperatures: A positive ice-albedo feedback. Geophysical Research Letters, 34 (6), doi: 10.1029/2006GL029021.