Water levels gauge installed in Grand Traverse Bay to assist in measuring glacial rebound

The glaciers of 10,000 years ago are still impacting the Great Lakes today.

Have you ever considered that the glaciers which formed the Great Lakes some 10,000 years ago are still having an impact today on Michigan’s Great Lakes shore? In fact, understanding glacial rebound is an active and ongoing area of research and application for scientists, surveyors, engineers, and riparians in 2015. The National Oceanic and Atmospheric Administration (NOAA), the federal agency which funds Michigan Sea Grant, is working with partners for the next several years to update the International Great Lakes Datum (IGLD) and has just recently installed a new self-contained microwave – satellite transmitted data water level gauge in West Grand Traverse Bay to help fill an information gap by collecting continuous water level information from May to September, 2015. NOAA was also able to install a comparable gauge in Munising, Michigan and Michigan City, Indiana for similar purposes after identifying these geographic areas as gaps in a recent report.

Scientists from four divisions of NOAA (the Center for Operational Oceanographic Products and Services, the National Geodetic Survey, the Office of Coast Survey and the Great Lakes Environmental Research Lab) travelled from Silver Spring, Maryland and Ann Arbor, Michigan to present a May 19 workshop to the public on Great Lakes water level fluctuations, sponsored and hosted by the Great Lakes Water Studies Institute at Northwestern Michigan College, Traverse City, Michigan. Northwestern Michigan College, a key collaborator and partner with Michigan State University Extension, recently received an integrated assessment planning grant from the Graham Sustainability Institute at the University of Michigan to get public input and host workshops during spring/summer 2015 on adaptive responses to Lake Michigan water level variations in the Grand Traverse Bay corridor.

The scientists explained the need for updating the IGLD1955/85: there is vertical crustal movement, typically uplift in the north of the Great Lakes, subsidence in the southern part of the Great Lakes, and new technology and precision satellite GPS accuracy now available. Technically known as glacial isostatic adjustment, it is a continuing process in which the Earth’s crust is warping in response to the melting of the last glacial ice sheets that crossed the area. In places where the glaciers were thicker and heavier (north of the Great Lakes), the crust of the earth is continuing to rise. The IGLD is essentially an Internationally agreed upon water level reference zero, over a defined time reference, and is a system of dynamic heights used for each of the Great Lakes and Lake St. Clair. The IGLD was originally established in 1955 (IGLD55), updated in 1985 (IGLD85), and data is being gathered to update it once again to IGLD 2020. This “reference zero” has also been codified into Michigan law (Natural Resources and Environmental Protection Act) and is used in various permitting decisions including use of the Great Lakes bottomlands and the ordinary high-water mark.

The last adjustment of the IGLD from 1955 to 1985 was significant: about 1 foot difference for Lake Superior and about a 0.7 foot for Lakes Michigan-Huron. Besides the typical net basin supply (such as inputs from upstream, runoff into the lake, precipitation over the lake, evaporation off the lake, and outputs downstream), the crustal movement of the earth from the glaciers is continuing to make a difference along the Great Lakes coastlines. It will be interesting to look at the data collected as part of the IGLD2020 update process and how the data contributes toward a better understanding our dynamic freshwater coasts.

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