Ft.

-1

0

+ 1

+ 2+ 3+ 4+ 5

M.

-0.3

0

+ 0.3

+ 0.6

+ 0.9+ 1.2+ 1.5

2010

AUG

1947

SEP

1947

OCT

1945

NOV

1945

DEC

1945

2011

JAN

1946

FEB

1952

MAR

1952

APR

1973

MAY

1973

JUN

1952

JUL

1947

AUG

19471934

SEP

19471934

OCT

19451934

NOV

19451934

DEC

19451934

2012

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

13

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

2010

AUG

1986

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2011

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

2012

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

13

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

2010

AUG

1986

SEP

1986

OCT

1986

NOV

1986

DEC

1986

2011

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

2012

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

13

JAN

19861936

CHART DATUM 572.3 FEET (174.4 METERS)

ELEVATIONS REFERENCED TO THE CHART DATUM OF EACH 

Superior

601.38

183.30

601.21

183.25603.08183.82

1950

600.26182.96

1926

602.07

183.51

MichHuron

577.56

176.04578.28

176.26

581.99177.39

1986

576.71

175.78

1964

579.33176.58

St.

Clair

573.95174.94574.84

175.21

577.20

175.93

1986

572.51

174.50

1934

574.77175.19

Erie

571.33174.14572.44174.48574.25175.03

1986

569.06

173.45

1934

571.88

174.31

Ontario

245.37

74.79

246.52

75.14

248.23

75.66

1947

243.24

74.14

1934

246.06

75.00

JULY MEAN LAKE LEVELS

(IGLD 1985)

* 2012 Ft.

M.

2011 Ft.

M.

** MAX.

Ft.

M.

Yr.

** MIN.

Ft.

M.

Yr.

** AVG. Ft.

M.

* provisional** Average, Maximum and Minimum for period 1918-2011

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).
 Information 

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 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 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. This information is available on the 

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

Great Lakes Basin Hydrology

July 2012

The entire Great Lakes basin received below average precipitation for the month of July. Over the past 12 months, 

Lakes Superior and Michigan-Huron have received below average precipitation. Lake Erie has received above average 

precipitation over the past year, while Lake Ontario has received below average precipitation. Precipitation over the 

entire Great Lakes basin is slightly below average over the past 12 months. In July, the net supply of water was below 

average for all the lakes. The tables below list July precipitation and water supply information for all Great Lakes 

basins. 

A comparison of monthly mean lake levels for July to long-term average (1918-2011) shows Lakes Superior and 

Michigan-Huron were 8 and 21 inches below average, respectively. Lake St. Clair was 10 inches below average in July, 

and Lakes Erie and Ontario were 7 and 8 inches, respectively, below average. Boaters should be aware of hazards to 

navigation due to continued below average water levels on all lakes. 

 

PRECIPITATION (INCHES) 

BASIN 

July 12-Month Comparison 

2012 Average Diff. % of 

Average 

Last 12 

months 

Average Diff. % of 

(1900-2008) (1900-2008) Average 

Superior 2.70 3.28 -0.58 82 28.70 30.51 -1.81 94 

Michigan-Huron 2.72 3.03 -0.31 90 30.62 32.44 -1.82 94 

Erie 2.81 3.39 -0.58 83 39.68 35.40 4.28 112 

Ontario 2.25 3.17 -0.92 71 32.26 35.71 -3.45 90 

Great Lakes 2.66 3.15 -0.49 84 31.73 32.64 -0.91 97 

 

 LAKE July WATER SUPPLIES1

 (cfs) July OUTFLOW2 (cfs) 

2012 

Average3

(1900-2008) 2012 

Average3

(1900-2008)

Superior 88,000 129,000 73,000 81,000 

Michigan-Huron 40,000 128,000 173,000 195,000 

Erie -14,000 7,000 193,000 209,000 

Ontario -4,000 24,000 233,000 261,000 

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

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