Ft.

-1

0

+ 1

+ 2+ 3+ 4+ 5

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9+ 1.2+ 1.5

2009

AUG

1947

SEP

1947

OCT

1945

NOV

1945

DEC

1945

2010

JAN

1946

FEB

1952

MAR

1952

APR

1973

MAY

1973

JUN

1952

JUL

1947

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

NOV

19451934

DEC

19451934

12

JAN

1946

1935

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

AUG

1986

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2010

JAN

1987

FEB

1987

MAR

1986

APR

1985

MAY

1986

JUN

1986

JUL

1986

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

NOV

1986

1934

DEC

1986

1934

12

JAN

19871935

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

AUG

1986

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2010

JAN

1986

FEB

1986

MAR

1986

APR

1986

MAY

1986

JUN

1986

JUL

1986

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

NOV

1986

1934

DEC

1986

1964

12

JAN

19861936

CHART DATUM 572.3 FEET (174.4 METERS)

ELEVATIONS REFERENCED TO THE CHART DATUM OF EACH 

Superior

601.21

183.25601.02183.19603.08183.82

1950

600.26182.96

1926

602.07

183.51

MichHuron

578.28

176.26

578.28

176.26

581.99177.39

1986

576.71

175.78

1964

579.33176.58

St.

Clair

574.84

175.21

574.44175.09

577.20

175.93

1986

572.51

174.50

1934

574.77175.19

Erie

572.44174.48571.75174.27574.25175.03

1986

569.06

173.45

1934

571.88

174.31

Ontario

246.52

75.14

245.96

74.97

248.23

75.66

1947

243.24

74.14

1934

246.03

74.99

JULY 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 Integrated Science Data Management, Department of Fisheries and Oceans, 

Canada. Historic and projected lake levels are derived by the Detroit District, U.S. Army Corps of Engineers and 

Environment Canada, 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,” twice monthly, 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). All of these publications can be accessed on the Internet at 

http://www.lre.usace.army.mil/glhh. 

Great Lakes Basin Hydrology

July 2011

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

received below average precipitation totaling 94% of average, while Lake Michigan-Huron received above average 

precipitation accumulating 107% of average for the month. Lake Erie received above average precipitation with 119% of 

average and Lake Ontario received about average precipitation at 99%. Over the past 12 months, precipitation on all of 

the lakes has been above average. During July, the net supply of water was above average for Lakes Superior and Erie, 

while the net supply was below average for Lakes Michigan-Huron and Ontario. Outflows from Lakes Superior and 

Michigan-Huron were below average in July while outflows from Lakes Erie and Ontario were above average. The tables 

below list July precipitation, water supply, and outflow information for the entire Great Lakes basin. 

A comparison of July monthly mean water levels to long-term (1918-2010) averages show that Lakes Superior and 

Michigan-Huron were 10 and 13 inches, respectively, below average. Lakes St. Clair, Erie, and Ontario were 1, 7 and 6 

inches above average, respectively. 

PRECIPITATION (INCHES) 

BASIN 

July 12-Month Comparison 

2011 Average Diff. % of 

Average 

 Last 12 

Months 

Average Diff. % of 

(1900-2008) (1900-2008) Average 

Superior 3.09 3.28 -0.19 94 32.55 30.51 2.04 107 

Michigan-Huron 3.23 3.03 0.20 107 35.02 32.44 2.58 108 

Erie 4.02 3.39 0.63 119 41.73 35.40 6.33 118 

Ontario 3.14 3.17 -0.03 99 37.33 35.71 1.62 105 

Great Lakes 3.29 3.15 0.14 104 35.52 32.64 2.88 109 

LAKE July WATER SUPPLIES1

 (cfs) July OUTFLOW2 (cfs) 

2011 

Average4

(1900-2008) 2011 

Average3

(1900-2008)

Superior 142,000 129,000 61,000 81,000 

Michigan-Huron 120,000 128,000 178,000 195,000 

Erie 11,000 7,000 220,000 209,000 

Ontario 9,000 24,000 296,000 261,000 

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

 Niagara and St Lawrence rivers average outflows are based on period of record 1900-1989 and 1900-2005, respectively 

4

Lakes Erie and Ontario average water supplies based on 1900-1989