Your chart has a datum date printed on it. I doubt the date is 1984. The lakes were on the rise then and peaked about 1985/ 6. I had to elevate the walkway on my boat well appx. 8" then. That walkway is about 36" lower today.
According to this site, Lake St. Clair datum is 572.3 feet above sea level and the current lake level is +18 inches above that. Now it also says that we are currently +16 inches over the lowest level recorded, so it seems that it's not based entirely on the lowest level ever seen on the lake.
I have 2 charts at work, one a St. Clair chart book dated Apr 1979 and another single chart from 1984. On each, under the main title for the chart it gives the projection for the chart, scale, soundings in feet, and a reference to "North American 1927 Datum" (not sure what that means.) Under the NOTES directly below that it says "Plane of Reference for this Chart (Low Water Datum).....571.7 ft." I think that's the key to the datum number. They have to have some plane of reference and it makes sense to have it at the low range, that way people will be forced to err on the safe side if they don't know the current levels. Charted bridge clearances are also in reference to the datum level, so it will be reduced by the 18" (currently) difference in the current levels.
While the website says we are +18"', it's actually +26" from the older charts I have at work. So to have any idea of actual levels, you need to know the current reported level and the datum level on the chart (or GPS software) you are referencing.