Information

Recorded monthly mean water levels in this bulletin are derived from a representative network of water level gages 

on each lake (see cover map). Providers of these data are the U.S. Department of Commerce, NOAA, National 

Ocean Service, and Integrated Science Data Management, Department of Fisheries and Oceans, Canada. The 

Detroit District, Corps of Engineers and Environment and Climate Change Canada derive historic and projected lake 

levels under the auspices of the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data.

This bulletin is produced monthly as a public service. The Corps also, on a weekly basis publishes online the Great 

Lakes, Connecting Channels and St. Lawrence River Water Levels and Depths, which provides a forecast of depths 

in the connecting rivers between the Great Lakes and the International Section of the St. Lawrence River. This 

Monthly Bulletin of the Lake Levels for the Great Lakes may be obtained free of charge by writing to the address 

shown on the front cover, by calling (313) 226-6441 or emailing hhpm@usace.army.mil. Notices of change of 

address should include the name of the publication. This information is available on the internet at 

https://www.lre.usace.army.mil/Missions/GreatLakesInformation.aspx.

Great Lakes Basin Hydrology

April 2021

Lake Superior received 127% of its monthly average precipitation while the other lakes received between 63 to 84%. 

The Great Lakes basin together received 82% of its monthly average precipitation. All lakes have been 11% to 20% 

below average over the past 12 months. Lakes Michigan-Huron, Erie, and Ontario received below average water 

supply for April while Lake Superior slightly exceeded its average supply. Outflows continue to be above average for

all lakes. 

Lakes Superior and Erie rose about 2 inches from March to April and Lake Ontario rose 5 inches. Lakes MichiganHuron and St. Clair remained near their levels from last month. The 6-month forecast projects all lakes to continue 

their seasonal rise.

PRECIPITATION (INCHES)

BASIN

April 12-Month Comparison

2021 Average Diff. % of 

Average

Last 12 

months 

Average Diff. % of 

(1900-2017) (1900-2017) Average

Superior 2.60 2.05 0.55 127 25.42 30.59 -5.17 83

Michigan-Huron 1.68 2.68 -1.00 63 28.98 32.52 -3.54 89

Erie 2.29 3.19 -0.90 72 28.31 35.55 -7.24 80

Ontario 2.49 2.95 -0.46 84 28.60 35.83 -7.23 80

Great Lakes 2.12 2.60 -0.48 82 27.72 32.76 -5.04 85

Lake

April WATER SUPPLIES1 (cfs) April OUTFLOW2 (cfs)

2021 Average3(1900-2008) 2021 Average3(1900-2008)

Superior 157,000 151,000 71,000 68,000

Michigan-Huron 163,000 286,000 231,000 182,000

Erie 30,000 68,000 246,000 207,000

Ontario 44,000 91,000 266,000 250,000

Notes: Values (excluding averages) are based on preliminary computations; cfs denotes cubic feet per second.

1 Net basin supply is the net result of precipitation falling on the lake, runoff from precipitation falling on the land which flows to 

the lake, and evaporation from the lake. Negative net basin supply denotes evaporation exceeded runoff and precipitation. 

The net total supply can be found by adding the net basin supply and the outflow from the upstream lake.

2 Does not include diversions.

3 Lake Ontario average water supplies and average outflows are based on period of record 1900-2005

 
Ft.

-1

0

+ 1

+ 2

+ 3

+ 4

+ 5

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9

+ 1.2

+ 1.5

2019

MAY

2017

JUN

2019

JUL

2019

AUG

1947

SEP

1947

OCT

1945

NOV

1945

DEC

1945

2020

JAN

1946

FEB

1952

MAR

1952

APR

1973

MAY

2017

1935

JUN

2019

1935

JUL

2019

1934

AUG

1947

1934

SEP

1947

1934

OCT

1945

1934

NOV

1945

1934

DEC

1945

1934

2021

JAN

1946

1935

FEB

1952

1936

MAR

1952

1935

APR

1973

1935

MAY

2017

1935

JUN

2019

1935

JUL

2019

1934

AUG

1947

1934

SEP

1947

1934

OCT

1945

1934

LAKE ONTARIO

CHART DATUM 243.3 FEET (74.2 METERS)

Ft.

-1

0

+ 1

+ 2

+ 3

+ 4

+ 5

+ 6

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9

+ 1.2

+ 1.5

+ 1.8

2019

MAY

2020

JUN

2019

JUL

2019

AUG

2019

SEP

2019

OCT

1986

NOV

1986

DEC

1986

2020

JAN

1987

FEB

2020

MAR

2020

APR

2020

MAY

2020

1934

JUN

2019

1934

JUL

2019

1934

AUG

2019

1934

SEP

2019

1934

OCT

1986

1934

NOV

1986

1934

DEC

1986

1934

2021

JAN

1987

1935

FEB

2020

1936

MAR

2020

1934

APR

2020

1934

MAY

2020

1934

JUN

2019

1934

JUL

2019

1934

AUG

2019

1934

SEP

2019

1934

OCT

1986

1934

LAKE ERIE

CHART DATUM 569.2 FEET (173.5 METERS)

Ft.

-1

0

+ 1

+ 2

+ 3

+ 4

+ 5

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9

+ 1.2

+ 1.5

2019

MAY

2020

JUN

2020

JUL

2019

AUG

2020

SEP

2020

OCT

1986

NOV

1986

DEC

1986

2020

JAN

2020

FEB

1986

MAR

2020

APR

2020

MAY

2020

1934

JUN

2020

1934

JUL

2019

1934

AUG

2020

1934

SEP

2020

1934

OCT

1986

1934

NOV

1986

1934

DEC

1986

1964

2021

JAN

2020

1936

FEB

1986

1926

MAR

2020

1934

APR

2020

1926

MAY

2020

1934

JUN

2020

1934

JUL

2019

1934

AUG

2020

1934

SEP

2020

1934

OCT

1986

1934

LAKE ST. CLAIR

CHART DATUM 572.3 FEET (174.4 METERS)

Ft.

-1

0

+ 1

+ 2

+ 3

+ 4

+ 5

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9

+ 1.2

+ 1.5

2019

MAY

2020

JUN

2020

JUL

2020

AUG

2020

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2020

JAN

2020

FEB

2020

MAR

2020

APR

2020

MAY

2020

1964

JUN

2020

1964

JUL

2020

1964

AUG

2020

1964

SEP

1986

1964

OCT

1986

1964

NOV

1986

1964

DEC

1986

2012

2021

JAN

2020

2013

FEB

2020

1964

MAR

2020

1964

APR

2020

1964

MAY

2020

1964

JUN

2020

1964

JUL

2020

1964

AUG

2020

1964

SEP

1986

1964

OCT

1986

LAKES MICHIGAN-HURON 1964

CHART DATUM 577.5 FEET (176.0 METERS)

Ft.

-1

0

+ 1

+ 2

M.

-0.3

0

+ 0.3

+ 0.6

2019

MAY

2019

JUN

2019

JUL

2019

AUG

2019

SEP

2019

OCT

1985

NOV

1985

DEC

1985

2020

JAN

2020

FEB

2020

MAR

1986

APR

1986

MAY

2019

1926

JUN

2019

1926

JUL

2019

1926

AUG

2019

2007

SEP

2019

2007

OCT

1985

1925

NOV

1985

1925

DEC

1985

1925

2021

JAN

2020

1926

FEB

2020

1926

MAR

1986

1926

APR

1986

1926

MAY

2019

1926

JUN

2019

1926

JUL

2019

1926

AUG

2019

2007

SEP

2019

2007

OCT

1985

1925

LAKE SUPERIOR

CHART DATUM 601.1 FEET (183.2 METERS)

ELEVATIONS REFERENCED TO THE CHART DATUM OF EACH RESPECTIVE LAKE

Superior

601.97

183.48

602.43

183.62

602.62

183.68

1986

599.48

182.72

1926

601.28

183.27

MichHuron

580.54

176.95

581.69

177.30

581.69

177.30

2020

576.15

175.61

1964

578.74

176.40

St.

Clair

575.92

175.54

577.13

175.91

577.13

175.91

2020

571.92

174.32

1926

574.34

175.06

Erie

572.93

174.63

574.31

175.05

574.31

175.05

2020

568.83

173.38

1934

571.65

174.24

Ontario

244.82

74.62

247.11

75.32

248.20

75.65

1973

242.88

74.03

1935

245.70

74.89

APRIL MEAN LAKE LEVELS

(IGLD 1985)

* 2021 Ft.

M.

2020 Ft.

M.

** MAX.

Ft.

M.

Yr.

** MIN.

Ft.

M.

Yr.

** AVG. Ft.

M.

* provisional

** Average, Maximum and Minimum for period 1918-2020

LEGEND

LAKE LEVELS

RECORDED

PROJECTED

AVERAGE **

MAXIMUM **

1985

1985

1973 1973

MINIMUM **

1936

1934

1926 1934

MAY 2021

US Army Corps

of Engineers

Detroit District

MONTHLY BULLETIN OF

LAKE LEVELS FOR THE

GREAT LAKES

Water levels for the previous year and the

current year to date are shown as a solid line 

on the hydrographs. A projection for the next

six months is given as a dashed line. This 

projection is based on the present condition 

of the lake basin and anticipated future 

weather. The shaded area shows a range of

possible levels over the next six months 

dependent upon weather variations. Current

and projected levels (solid and dashed lines) 

can be compared with the 1918-2020 average

levels (dotted line) and extreme levels 

(shown as bars with their year of occurrence).

The legend below further identifies the 

information on the hydrographs.

The levels on the hydrographs are shown in

both feet and meters above (+) or below (-) 

Chart Datum. Chart Datum, also known as 

Low Water Datum, is a reference plane on 

each lake to which water depth and Federal 

navigation improvement depths on navigation 

charts are referred.

All elevations and plots shown in this bulletin 

are referenced to International Great Lakes 

Datum 1985 (IGLD 1985). IGLD 1985 has its 

zero base at Rimouski, Quebec near the mouth 

of the St. Lawrence River (approximate sea 

level).