Ft.

-1

0

+ 1

+ 2+ 3+ 4+ 5

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9+ 1.2+ 1.5

2009

MAY

1973

JUN

1952

JUL

1947

AUG

1947

SEP

1947

OCT

1945

NOV

1945

DEC

1945

2010

JAN

1946

FEB

1952

MAR

1952

APR

1973

MAY

19731935

JUN

19521935

JUL

19471934

AUG

19471934

SEP

19471934

OCT

19451934

NOV

19451934

DEC

19451934

2011

JAN

1946

1935

FEB

1952

1936

MAR

19521935

APR

19731935

MAY

19731935

JUN

19521935

JUL

19471934

AUG

19471934

SEP

19471934

OCT

19451934

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

2009

MAY

1986

JUN

1986

JUL

1986

AUG

1986

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2010

JAN

1987

FEB

1987

MAR

1986

APR

1985

MAY

1986

1934

JUN

1986

1934

JUL

1986

1934

AUG

1986

1934

SEP

1986

1934

OCT

1986

1934

NOV

1986

1934

DEC

1986

1934

2011

JAN

19871935

FEB

1987

1936

MAR

1986

1934

APR

19851934

MAY

1986

1934

JUN

1986

1934

JUL

1986

1934

AUG

1986

1934

SEP

1986

1934

OCT

1986

1934

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

2009

MAY

1986

JUN

1986

JUL

1986

AUG

1986

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2010

JAN

1986

FEB

1986

MAR

1986

APR

1986

MAY

1986

1934

JUN

1986

1934

JUL

1986

1934

AUG

1986

1934

SEP

1986

1934

OCT

1986

1934

NOV

1986

1934

DEC

1986

1964

2011

JAN

19861936

FEB

19861926

MAR

1986

1934

APR

19861926

MAY

1986

1934

JUN

1986

1934

JUL

1986

1934

AUG

1986

1934

SEP

1986

1934

OCT

1986

1934 CHART DATUM 572.3 FEET (174.4 METERS)

ELEVATIONS REFERENCED TO THE CHART DATUM OF EACH 

Superior

600.10

182.91

600.66

183.08602.62183.68

1986

599.48182.72

1926

601.25

183.26

MichHuron

577.13

175.91

577.79

176.11

581.46

177.23

1986

576.15

175.61

1964

578.71

176.39

St.

Clair

573.79174.89573.82

174.90

576.84175.82

1986

571.92174.32

1926

574.28175.04

Erie

571.52

174.20

571.16

174.09574.08174.98

1985

568.83173.38

1934

571.59174.22

Ontario

245.57

74.85

244.95

74.66

248.20

75.65

1973

242.88

74.03

1935

245.67

74.88

APRIL MEAN LAKE LEVELS

(IGLD 1985)

* 2011 Ft.

M.

2010 Ft.

M.

** MAX.

Ft.

M.

Yr.

** MIN.

Ft.

M.

Yr.

** AVG. Ft.

M.

* provisional** Average, Maximum and Minimum for period 1918-2010

1985

1985

1973 1973

1936

1934

1926 1934

LAKE ONTARIO

LAKE ERIE

LAKE ST. CLAIR

LEGEND

LAKE LEVELS

RECORDEDPROJECTEDAVERAGE **MAXIMUM **MINIMUM **

The levels on the hydrographs are shown inboth 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).
Recorded 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 National Ocean Service, National Oceanic and Atmospheric Administration, 

U.S. Department of Commerce, and the Marine Environmental Data Service, Department of Fisheries and Oceans, 

Canada. The Detroit District, Corps of Engineers and Environment 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. Tables of possible storm-induced rises at key locations on the 

Great Lakes are available on request. The Corps also publishes the "Great Lakes, Connecting Channels and St. 

Lawrence River Water Levels and Depths,” weekly, which provides a forecast of depths in the connecting rivers between 

the Great Lakes and the International Section of the St. Lawrence River. These publications can be obtained free of 

charge by writing to the address shown on the front cover, or by calling (313) 226-6441. Notices of change of address 

should include the name of the publication(s). The Internet address http://www.lre.usace.army.mil/glhh also contains 

this information. 

Great Lakes Basin Hydrology

April 2011

Overall, the Great Lakes basin experienced above average precipitation during the month of April. Lakes Superior, 

Michigan-Huron, Erie, and Ontario received 180%, 209%, 184%, and 186% of average precipitation, respectively. During 

the past 12 months, precipitation has been near average for Lake Superior and above average for all of the other Great 

Lakes. Outflows from Lakes Superior and Michigan-Huron were below average in April while the outflows from Lakes 

Erie and Ontario were near average. The tables below list April precipitation, water supply, and outflow information for 

the entire Great Lakes basin. 

The above average precipitation last month caused all of the Great Lakes water levels to rise quicker than average in 

April. Comparison of April monthly mean water levels to long-term average shows Lakes Superior, Michigan-Huron, and 

St. Clair were 14, 19, and 6 inches below average, respectively, while Lakes Erie and Ontario were both 1 inch below 

average.

PRECIPITATION (INCHES) 

BASIN 

April 12-Month Comparison 

2011 Average Diff. % of 

Average 

Average 

Last 12 

months 

Average Diff. % of 

(1900-2008) (1900-2008) Average 

Superior 3.62 2.01 1.61 180 31.74 30.51 1.23 104 

Michigan-Huron 5.43 2.60 2.83 209 36.12 32.44 3.68 111 

Erie 5.80 3.16 2.64 184 42.53 35.40 7.13 120 

Ontario 5.41 2.91 2.50 186 41.25 35.71 5.54 116 

Great Lakes 4.99 2.54 2.45 196 36.00 32.64 3.36 110 

Lake 

April WATER SUPPLIES1

 (cfs) April OUTFLOW2 (cfs) 

2011 

Average4

(1900-2008) 2011 

Average3

(1900-2008)

Superior 160,000 150,000 52,000 68,000 

Michigan-Huron 433,000 284,000 166,000 182,000 

Erie 128,000 67,000 209,000 207,000 

Ontario 125,000 93,000 259,000 251,000 

Notes: Values (excluding averages) are based on preliminary computations. CFS denotes cubic feet per second. 1

 Negative water supply denotes evaporation from lake exceeded runoff from local basin. 2

 Does not include diversions. 

3

 St Lawrence River average outflow is based on period of record 1900-2005 4 Lake Ontario average water supply based on 1900-1989