Monthly lake-wide average water levels (1918 - Present)

Water levels are recorded at 53 monitoring stations in the U.S. and 33 stations in Canada. Data are available at a variety of timescales (6-minute average, hourly, daily, monthly). In 1992, the Coordinating Committee on Great Lakes Basic Hydraulic and Hydrologic Data approved a set of gauges (U.S. and Canadian) for each lake that water resource professionals believe give the most accurate reflection of the lake's overall water level when averaged. These lake-wide average levels are used by the Army Corps of Engineers and Environment Canada in their forecast bulletins. These data are available starting in 1918 because before that time there were too few gauges to calculate a realistic lake-wide average.

The gauge networks are listed below:

Lake Superior	Lake Michigan-Huron	Lake St. Clair	Lake Erie	Lake Ontario
Duluth, MN	Ludington, Mackinaw City, Harbor Beach, MI	St. Clair Shores, MI	Toledo, Cleveland, Fairport, OH	Rochester, Oswego, NY
Marquette C.G., Pt Iroquois, MI	Milwaukee, WI	Belle River, ON	Port Stanley, Port Colborne, ON	Port Weller, Toronto, Cobourg, Kingston, ON
Michipicoten, Thunder Bay, ON	Thessalon, Tobermory, ON

Current month provisional averages from Coordinated Gauges

These data are produced by an automated script twice daily. The script retrieves the current conditions water levels PDFs found here from the USACE Detroit District webpage. After retrieving, the script parses the text of the PDFs, finding the daily averages and current average for the month, and writing each lake's daily and monthly averages to text files for display on the dashboard. When the PDF has complete daily data, the average for the month is appended to the monthly data set automatically. As this is a provisional average for the month, the set of monthly water level averages will be checked annually after final coordination of the year's worth of data.

Earlier monthly water levels: 1860-1917

Before 1918, there were very few water level gauges in the Great Lakes. However, Superior, Michigan, Huron, Erie, and Ontario all have at least one water level gauge that has been in operation since 1860. Because of isostatic rebound (shifting of the land surface following the retreat of the glaciers), the levels at these gauges may not represent the lake-wide average level if they are far from the lake's outlet. These historical levels can be adjusted to represent the levels at the outlet. You may compare the adjusted levels (1860-1917) to the original measurement on Superior and Erie by viewing the master gauge and the adjusted monthly average time series (diamonds and triangles respectively). The equations used to adjust these early gauge records are:

Lake	Gauge	Correction
Superior	Marquette	To Pt Iroquois: PI = -.00108382*year+2.153 + Marquette
Michigan-Huron	Harbor Beach	None
St. Clair	Gross Pointe Yacht Club	None
Erie	Cleveland	To Buffalo: Buff = -.001008382*year+2.153 + Cleveland
Ontario	Oswego	None

Master Gauge Station History

In several cases the master gauge has changed since 1860, either due to a relocation or a change in conditions causing a gauge to lose stability. The table below summarizes the history of master gauge stations on the Great Lakes according to NOS/CO-OPS:

Lake	Station	Time Period
Superior	Marquette, MI	1860 - Sep 1980
	Marquette C.G., MI	Oct 1980 - Present
Michigan-Huron	Harbor Beach, MI	1860 - Present
St. Clair	Gross Pointe Yacht Club, MI	1898 - Feb 1968
	St. Clair Shores, MI	March 1968 - Present
Erie	Cleveland	1860 - Dec 1991
	Fairport	Jan 1992 - Present
Ontario	Oswego	1860 - Present

Per Month Records

Per month averages and record highs and lows for each month are available for the lake-wide average water levels (1918-2016). These values are coordinated by the U.S. Army Corps of Engineers and Environment Canada. They may be found here, along with additional water level data.

Low Water Datum

The low water datum (or 'chart datum') is the reference plane accepted as the base elevation for each of the Great Lakes. Depths published on navigational charts are relative to low water datum. This plane is reestablished approximately every 30 years due to glacial isostatic adjustment, the 'bounce back' of the earth's crust in response to the weight of the glaciers.

Note: These values have been established in both meters and feet using specific rounding procedures. The dashboard is using the accepted values in both units.

What is the IGLD85?

IGLD85 refers to the International Great Lakes Datum, an elevation benchmark against which all water level gauging stations in the Great Lakes are compared. This reference point was last established in 1992.