Ordinary High Water Mark (OHWM)

The ordinary high water mark (OHWM) is defined in the Part 325 administrative rules as follows:

(j) "Ordinary high water mark" means the elevations set by the act. When the soil, configuration of the surface, or vegetation has been altered by man's activity, the ordinary high water mark shall be located where it would have been if this alteration had not occurred

The OHWM is the elevation along the shoreline where a construction permit under Part 325 is required for such activities as dredging, seawalls, rock revetments, permanent docks and other structures. The elevations are listed in Section 32502 of Part 325. The elevations are referenced upon the baseline datum set called International Great Lakes Datum 1955 (IGLD 55).

Please note that water levels can extend above these Ordinary High Water Marks of the Great Lakes. A Part 325 Construction Permit is recommended for work in the water when water levels are above the Ordinary High Water Mark Elevations for each of the Great Lakes. 

These elevations have since been revised to a new datum set based upon studies conducted by the U.S. Army Corps of Engineers in the mid-1980s now called IGLD 1985. These revised elevations are shown here:

Great Lakes Ordinary High Water Mark example illustration