The ordinary high water mark (OHWM) is defined in the Administrative Rules of Part 325 as the elevations set by the act.  Along and below the OHWM elevations, permits under Part 325 are required for activities such as dredging, filling, seawalls, docks, and other structures.  These elevations are set in Section 32502 of Part 325.

Great Lakes Ordinary High Water Mark example illustration

The elevations are referenced to a baseline datum set called the International Great Lakes Datum 1955 (IGLD 55). The elevations were updated to a revised datum called IGLD 85 in 1985 by the US Army Corps of Engineer. The IGLD is again in the process of being updated and these elevations will change again in the future. The 1955 and 1985 elevations are shown here:

You may also hear about the natural OHWM. The Natural OHWM is the point on the land where the presence of water is so continuous that it leaves a distinct mark such as a change in soil type or presence and absence of plants. The elevation based OHWM set in Part 325 may not always be at the same location as the natural OWHM. Water levels on the Great Lakes can extend above both of the OHWM’s. EGLE recommends contacting the staff person that covers your county any time you plan to conduct work on the beach or in the water of the Great Lakes to ensure compliance with Part 325 and other Parts of the Natural Resources and Environmental Protection Act.