Volume 210: 2021 Annual Summary

Background

The U.S. Army Corps of Engineers (USACE) 

monitors and forecasts the water levels of each of 

the Great Lakes. This report is primarily focused 

on summarizing the hydrologic conditions of the 

Great Lakes basin in 2021.

Official water levels are based on monthly lakewide means, and the period of record used for each 

of the lakes includes the years 1918 to 2020. These 

data have been coordinated between the United 

States and Canada. 

The elevations used are referenced to the 

International Great Lakes Datum of 1985. The 

water level of each lake is averaged from a 

network of individual gages around each lake. 

Also of note is that Lake Michigan and Lake 

Huron are hydraulically treated as one lake due to 

their connection at the Straits of Mackinac.

The Great Lakes are very large and behave 

differently from smaller, inland lakes. In general, 

Great Lakes water levels do not rise and fall 

substantially with individual storms. Significant 

water level fluctuations require multiple months, 

seasons, or years of wet or dry conditions.

Seasonal changes in weather patterns typically 

cause an annual pattern of rising and falling of 

Great Lakes water levels. Each of the Great Lakes 

exhibits a seasonal rise in the spring primarily 

caused by an increase in liquid precipitation, 

increased runoff due to melting of accumulated 

snow, and low evaporation rates. The typical 

seasonal decline of the water levels occurs during

the fall and winter, and is primarily caused by an 

increase in evaporation, a decrease in 

precipitation, and the accumulation of snowpack 

on the land area.

The Net Basin Supply (NBS) is an important 

quantity for understanding the amount of water 

which arrives to the lake. USACE uses the 

residual method to compute NBS, shown below in 

the summarizing equations: 

Residual Method Net Basin Supply:

NBS = WL – I – D + O

WL: Water Level Change

I: Connecting Channel Inflow

O: Connecting Channel Outflow

D: Diversion into(+) or out(-) of lake

Altogether, NBS represents the combined effects 

of precipitation over the lake, runoff to the lake, 

and evaporation off of the lake. NBS is the main 

driver of water levels and is also discussed in more 

detail in the following sections for each lake. All 

2020 and 2021 NBS data discussed in this 

publication is preliminary. Long-term average 

(LTA) NBS is based on the coordinated period of 

record that spans from 1900 to 2008.

2021: Transition from Record High Levels

After two consecutive years with record high water 

levels in 2019 and 2020, 2021 saw water levels 

transition below record high levels. Lake Superior 

and Lake Ontario even experienced portions of the 

year below LTA levels. The last time Lake 

Superior’s water level was below LTA levels was 

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Update
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in the spring of 2014 and the last time the water 

level was below LTA levels on Lake Ontario was 

the fall of 2018. Conditions varied throughout the 

Great Lakes basin in 2021. The first half of 2021 

experienced drier conditions and was associated 

with the lakes experiencing a larger than average 

seasonal decline and a smaller than average 

seasonal rise (Table 1). 

Table 1. Seasonal Declines and Rises in 2020-

2021. 

Lake Seasonal

Decline

Seasonal

Rise

Average

Superior 15” 6” 12”

Mich-Hur 21” 4” 12”

St. Clair 22” 10” 16”

Erie 21” 9” 14”

Ontario 34” 14” 21”

* Seasonal declines calculated from seasonal peak in 2020 to 

seasonal low in 2021. Rises calculated from seasonal low to 

seasonal peak in 2021. Based on monthly mean water levels.

However, in the summer conditions transitioned 

from generally dry to generally wet in the Lakes 

Michigan-Huron, St. Clair, Erie, and Ontario 

basins. The Lake Superior basin continued to 

experience drier than normal conditions and water 

levels eventually fell below LTA levels. Figure 1 

shows precipitation as percent of normal in the 

Spring and Summer of 2021. It indicates the 

transition from a dry spring throughout the Great 

Lakes region, to a wetter summer in central and 

eastern portions of the basin. 

Figure 1. Percent of Normal Precipitation 

during the Spring (left) and Summer (right) 

Source: Midwest Regional Climate Center

Lake Superior

Figure 2 shows the 2021 water levels of Lake 

Superior compared with the 2020 levels and the 

long-term average (LTA) levels. A similar figure 

is shown for each lake in the following sections. 

Each lake's data has been plotted relative to the 

chart datum (a.k.a. low water datum). Chart datum 

for Lake Superior is 601.1 feet. The black line in 

Figure 2 represents the daily 2021 water levels and 

the dashed purple line shows the 2020 levels. The 

LTA levels for each month are shown as dots and 

maximums and minimums are represented as 

black dashes. Figure 3 shows monthly residual 

NBS, and the NBS is shown as inches over water 

surface. The 2021 NBS is compared to 2020 and 

the LTA for each month. 

The Lake Superior basin saw above average 

evaporation and below average precipitation and 

runoff for the majority of the year, leading to 

below average NBS in eight of the twelve months 

of 2021. Lake Superior started 2021 with a 

January monthly mean level of 602.17 feet, about 

6.5 inches lower than the record-high January level 

set in 2020 and 8 inches higher than the LTA 

January level. Lake Superior was below its 

monthly mean record high levels for all months in 

2021. 

Figure 2. Lake Superior Water Levels
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Figure 3. Lake Superior Residual Net Basin 

Supply compared to Long-Term Average

(LTA)

Lake Superior reached its peak monthly average 

level of 602.23 feet in July. The lake was higher 

than its LTA by up to 8 inches from January to 

August and fell below its LTA for the remaining 

months. Lake Superior rose 6 inches compared to 

its 2020 rise of 9 inches. 

As a result of the persistent dry conditions in the 

basin, water levels transitioned from above LTA 

levels to below LTA levels in the summer. Lake 

Superior’s monthly mean levels were above their 

LTA levels in the months of January through July. 

In August, Lake Superior matched its LTA level 

and by September, the monthly average level fell 

below the monthly LTA level for the first time 

since March 2014, a 93-month period, and levels 

remained below the LTA for the rest of the year. 

The lake began the year 8 inches above its LTA 

and ended the year 4 inches below its LTA. 

Lake Superior received less than average rainfall 

in all months except April and December and 

ended the year having received only 80% of yearly 

average precipitation. The greatest amount of 

monthly precipitation was 3.43 inches in 

September, just under the monthly average. The 

least amount of monthly precipitation was 0.55 

inches in January, well below the monthly average

at about 29% of average. The lack of precipitation 

also led to drought conditions to develop and 

persist throughout 2021 in the Lake Superior 

basin. Figure 4 shows the North American 

Drought Monitor at the end of August, when 

Extreme Drought Conditions were established in 

northwestern portions of the basin. 

Figure 4. North American Drought Monitor 

Map for the end of August 2021

Lake Michigan-Huron

Figure 5 shows the 2021 water levels of Lake 

Michigan-Huron compared with the 2020 levels 

and the LTA levels. The Lake Michigan-Huron 

basin saw below average precipitation and above 

average evaporation for the majority of the year, 

with average to below average runoff. This 

combined led to below average NBS in seven of 

the twelve months of 2021. The Lake MichiganHuron basin received less than average rainfall in 

eight of the twelve months, with only June, July, 

August, and October precipitation levels 

exceeding the average, and overall received 86% 

of yearly average precipitation. The greatest 

amount of monthly precipitation received was 4.35 

inches in July, which was 144% of the July 

monthly average. The least amount monthly was 

0.73 inches in January, which was only 34% of the 

January monthly average. 
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Figure 5. Lake Michigan-Huron Water Levels

Lake Michigan-Huron experienced its peak level 

for the year in January, which is a rare occurrence, 

and only the 4th time it has occurred in our period 

of record of monthly mean water levels going back 

to 1918. For the rest of the year, lake levels 

generally declined, with a slight seasonal rise 

during the middle of the year. 2021 began with a 

January monthly mean level of 580.94 feet, about 

7.5 inches below the record-high January level set 

in 2020 and almost 30 inches higher than the LTA 

January level. The lowest monthly mean level of 

579.69 feet was seen in December, which was 13 

inches above the LTA December level and 22.5 

inches lower than the record-high December level 

set in 1986. 

Figure 6. Lake Michigan-Huron Residual Net 

Basin Supply compared to LTA

Lake Michigan-Huron was above its LTA monthly 

mean levels for all months in 2021. Lake 

Michigan-Huron rose 4 inches compared to its 

2020 rise of about 9 inches. The monthly residual 

NBS for Lake Michigan-Huron was near average 

in June and above average in July, August, 

October, and December, with the remaining 

months below average (Figure 6). 

Lake St. Clair

Lake St. Clair was below record high levels for the 

duration of 2021. However, lake levels remained 

well above LTA levels. The lake has been above 

its monthly LTA level every month since March 

2015, an 82-month period. 

Lake St. Clair’s January monthly mean level of 

576.18 feet was 7 inches below the record high 

level from January 2020 and by February the water 

level (575.69 ft.) was 13 inches below its record 

high level from February 1986. The water level 

graphic (Figure 7) shows that there was a large 

drop in the daily water levels during the month of 

February. This was a result of an ice jam in the St. 

Clair River. 

Figure 7. Lake St. Clair Water Levels 

An example of an ice jam impacting the St. Clair 

River is shown in Figure 8. Ice restrictions in the 

St. Clair River cause the flow of water to be 

reduced into Lake St. Clair. When an ice jam or 
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restriction occurs, water levels will rise upstream 

of the restriction and water levels will drop 

downstream of the restriction. By the end of 

February, water levels had generally recovered 

from the ice jam. 

Figure 8. Example of ice restriction impacting 

the St. Clair River

The spring was relatively dry in the Lake St. Clair 

basin with April and May NBS below average 

(Figure 9). A time of year when water levels would 

typically rise significantly, the monthly mean 

water level from March to June only rose 3 inches, 

when on average it would rise 11 inches during this 

time. However, a wetter summer and early fall, 

allowed for water levels to see a later rise. By the 

end of the year, the December monthly mean level 

was 15 inches below the record high from 1986 

and 8 inches below the level from 2020. 

Figure 9. Lake St. Clair Net Basin Supplies

compared to LTA 

Lake Erie

After setting record high water level marks in 2019 

and 2020, water levels on Lake Erie remained 

below record high levels for the entire calendar 

year of 2021. However, water levels continued to 

be above average and have remained above their 

monthly LTA levels since June 2015, a period of 

79 consecutive months. In the beginning of 2021, 

water levels on Lake Erie were continuing their 

seasonal decline, reaching its seasonal low 

monthly mean level in February (Figure 10). 

Figure 10. Lake Erie Water Levels

The beginning of 2021 was rather dry across the 

Lake Erie basin and Figure 11 shows that the NBS 

from January through May was below the LTA. 

This contributed to the large seasonal decline and 

marginal seasonal rise experienced on the lake 

(Table 1). However, toward the end of June 

conditions became much wetter in the Lake Erie 

basin and NBS was above average in all three 

summer months (June, July, August). In July, the 

basin received close to 5 inches of rain, leading to 

well above average NBS and a rise in water level. 

Although September saw a return to drier 

conditions, October brought over 5 inches of 

precipitation (~180% of average) to the region, 

which combined with above normal runoff 

brought NBS again well above average. 
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Figure 11. Lake Erie Residual Net Basin 

Supply compared to LTA

As a result of the wet October, the September and 

October monthly mean water levels for Lake Erie 

were the same at 573.06 feet. On average, between 

September and October the water level typically 

declines about 4 inches. The wetter October also 

brought water levels close to the water levels of 

2020. By the end of 2021, the December 2021 

mean level was 3 inches below the level from 2020 

and 12 inches below the record high set in 1986. 

Lake Ontario

By the end of 2020, water levels on Lake Ontario 

were approaching LTA levels. At the beginning of 

2021, the January mean level of 244.82 feet, was 

just 2 inches above average (Figure 12). The first 

6 months of the year generally experienced NBS 

below average (Figure 13), which was influenced 

by the lack of precipitation and increased 

evaporation. These conditions contributed to the 

above average seasonal decline and below average 

seasonal rise on the lake (Table 1). From the peak 

in May of 2020 to the seasonal low in March of 

2021, the lake declined 34 inches, when an average 

seasonal decline from the seasonal peak to valley 

is typically about 21 inches. 

Figure 12. Lake Ontario Water Levels

From February to August, water levels were below 

LTA levels on Lake Ontario. However, the second 

half of 2021, was much wetter than the first half. 

NBS was above average the last 6 months of the 

calendar year (Figure 13). Similarly, to Lake Erie, 

Lake Ontario experienced exceptionally wet 

months of July and October. Over 5 inches of 

precipitation fell in each month, which is more 

than 2 inches above average for July and October. 

Runoff was also increased during the last half of 

the year, due to the increased precipitation. 

Although the seasonal peak monthly mean level 

occurred in August, the water level only declined 

for about two months before it began to rise again. 

The wet conditions impacted this rise in water 

level and in September Lake Ontario’s level 

returned to being slightly above average. The 

water level rose between October and December 

by 3.5 inches, when on average the water level 

typically declines 3.5 inches. Also, the December 

monthly mean level was the highest monthly mean 

level experienced in 2021. This is rare and in our 

period of record of monthly mean water levels it’s 

only the second time this has occurred, with the 

previous occurrence in 1926. 
Vol. 210 Great Lakes Update February 2022 

Page 7

Figure 13. Lake Ontario Residual Net Basin 

Supply compared to LTA

Outreach

Prior to the pandemic, several face-to-face 

meetings were occurring because of the record 

high water levels. Throughout the pandemic over 

the last two years, the Hydraulics and Hydrology 

Branch has continued to give virtual outreach 

presentations on basin conditions and water levels.

As we move forward into an uncertain future, we 

plan to continue providing outreach presentation in 

either virtual or face-to-face settings. If your group 

is interested in learning more about Great Lakes 

water levels and basin conditions, please reach out 

to us using the contact information at the end of the 

article.

Also, information related to Great Lakes basin 

conditions, including precipitation, runoff, 

evaporation, and NBS can be found on our website 

at the following link under the header “Recent Net 

Basin Supply Conditions”.

https://www.lre.usace.army.mil/Missions/Great-LakesInformation/Great-Lakes-Information-2/Basin-Conditions/

Also at that link, there are hydrology and water 

level summary reports that can be found at the top 

of the page. The hydrology summaries provide 

information about the current state of basin 

conditions and water levels. The water level 

summaries include information about the 6-month 

water level forecast. 

Water level data is also accessible on our website 

at the following link:

https://www.lre.usace.army.mil/Missions/Great-LakesInformation/Great-Lakes-Information-2/Water-Level-Data/

Please note that our daily water level and 

connecting channel reports will be changing 

formats soon. The new versions are available in an

HTML format and the PDF versions will soon be 

discontinued. Our mission continues to be to 

provide high quality hydrologic and hydraulic data 

to our customers and the updated reports will serve 

that mission well. 

“On the Level” Video Series

The “On the Level” video series was launched in 

the summer of 2021 to share more water level 

information with our stakeholders. The video

series provides us an avenue to reach people across 

the basin with useful information about various 

Great Lakes water level topics. There have been 6 

short educational videos released since this 

summer, with more to come in 2022! Topics of the 

first six videos were related to the following: 

• Current and forecasted water level 

conditions 

• How to Interpret the 6-month forecast 

graphic 

• Lake Superior Regulation 

• Flow Measurements on the Detroit River 

• Water Level Future Scenarios Product

Please check out our first 6 videos on our District’s 

YouTube page at the following link: 

https://www.youtube.com/playlist?list=PLqtbMF

yAaYYNkKS2wxdyBsDYSBQXP3HLq
Vol. 210 Great Lakes Update February 2022 

Page 8

Figure 14: Recording “On the Level” video in 

July 2021 (USACE Detroit District). 

More Great Lakes Information

Update articles are included periodically in the 

Monthly Bulletins highlighting topics and 

explanations relevant to Great Lakes water levels. 

Nearly all past Update Articles dating back to 1985 

can be accessed here:

https://www.lre.usace.army.mil/Missions/Great-LakesInformation/News-and-Information/Great-Lakes-UpdateArticles/

The Monthly Bulletin is sent by postal mail. To be 

added to the postal mailing list, please send an 

email to hhpm@usace.army.mil or call 1-888-694-

8313 and select option 1. Alternatively, the 

Monthly Bulletin can be viewed on our website. 

The home page is: http://www.lre.usace.army.mil. 

In addition to the Monthly Bulletin, the Detroit 

District issues the Weekly Great Lakes Water Level 

Update and the Weekly Great Lakes Connecting 

Channels Water Levels and Depths. Both products 

are updated at the end of each week. All forecast 

products are located here:

https://www.lre.usace.army.mil/Missions/Great-LakesInformation/Great-Lakes-Water-Levels/Water-LevelForecast/

Additionally, a product called Great Lakes Water 

Level Future Scenarios illustrates outcomes that 

would occur under historical weather and water 

supply conditions, with scenarios chosen based on 

similarities to recent conditions. This product is 

updated every three months and is located here:

https://www.lre.usace.army.mil/Missions/Great-LakesInformation/Great-Lakes-Water-Level-Future-Scenarios/

The Detroit District also has a Facebook page 

which can be found here: 

https://www.facebook.com/USACEDetroitDistrict/

Please email questions and comments to 

hhpm@usace.army.mil. To contact the District by 

phone call toll free 1-888-694-8313 and select 

option 1. The Detroit District’s mailing address is:

Department of the Army

Detroit District Corps of Engineers

ATTN: CELRE-CHW

477 Michigan Avenue 

Detroit, MI 48226