We’re on the rebound

You might know that the average benchmark elevation for Lake Superior is 601.1 feet above sea level.

But did you know where sea level is measured? It’s at Rimouski, Quebec.

Periodic recalculations of the elevation reference system — known as the International Great Lakes Datum (IGLD) — have to be made because of the movement of the earth’s crust. The last adjustment of the IGLD was in 1985.

A word about the movement of the earth’s crust, as described in Living with the Lakes, published by the U.S. Army Corps of Engineers in 1999:

The glaciers of the last Ice Age were thousands of miles thick in places and weighed heavily upon the earth. As they melted, the earth began to rise — and continues to rise today. The northeastern corner of the Lake Superior basin, where the glacier pack was thickest and heaviest, rises by as much as 21 inches per century. On the southern parts, though, where there was little glacial influence, there is no crustal movement (also known as isostatic rebound). As a result, the Great Lakes basin is gradually tipping.

This rebound does not affect the amount of water in a lake, but does affect the levels of water. That is why water levels are higher today in Duluth and lower at Michipicoten, Ontario, on the opposite side of the lake than they were several decades ago.

It’s also why periodic review of benchmarks is necessary. The current datum is expected to be good for about another 20 years. Then we’ll know if Duluth again has moved closer to the heavens.