L A K E L E V E L S
NEW WATER STANDARDS
S
ince the last ice age, the earth's crust beneath the Great Lakes has been expanding - rebounding from the weight of glaciers. After all, the Great Lakes were formed by glaciers. As the National Oceans and Atmospheric Administration (NOAA) puts it, the massive weight and move- ment of the mile-thick Laurentide ice sheet "gouged out the earth to form the lakes' basins." As the glacier melted, the deepened areas filled with water, becoming lakes. Rebounding throughout North America and the Great Lakes/St. Lawrence Seaway region is occurring gradually - and to a greater extent in the Upper Lakes. For example, in one 30-year period, measurements showed a 12-inch rise in the lakebed of Lake Superior. As a result, U.S. and Canadian government representatives from a variety of agencies assemble about every 30 years to measure and update the International Great Lakes Datum (IGLD). The 2020 update - the first since 1985 - is underway. "We're on schedule to produce that next update," says John Allis, Chief, Great Lakes Hydraulics and Hydrology Office, U.S. Army Corps of Engineers. "We expect to release it in 2025." Allis is working with the coordinat- ing committee on Great Lakes Basic Hydraulic & Hydrologic Data, the formal name of the binational group. Committee leadership is provided by NOAA for the U.S. and the Canadian Hydrographic Service. The regional adjustments in the earth's crust differ because of the early variations in glacial thickness, move- ment and melting, according to Terese Herron, a hydrographer with Fisheries and Oceans Canada who answered questions with assistance from represen- tatives of the coordinating committee.
Taking Measurements
The new IGLD datum will, for the first time, be set using state-of-the-art equip- ment which measures more accurately and faster. It also creates modeling that sets benchmarks. The technology includes use of satellite systems, like GPS. On the ground, the committee is col- lecting data from permanent and tempo- rary water level gauges. In the Great Lakes region, more than 200 permanent and seasonal gauges - in U.S. and Canadian waters - are being used to gather data. "These will densify the water level measurement network in an effort to determine the variations in the surfaces of each of the Great Lakes," according to the committee. In 1955 and 1985, the IGLD was determined by people physically traveling around the Lakes and using poles and levels. It was expensive and time con- suming. Leveling is susceptible to more errors than the new modeling, which allows for detailing down to centimeters. The new survey techniques will collect data in about six weeks instead of years and millions of dollars spent on crews manually moving about the basin. This time, preparation coincides with the binational effort to update datum throughout North America.
UPDATING INTERNATIONAL GREAT LAKES DATUM TO REFLECT LAKE BOTTOM REBOUND
Estimated Timeline for LWD Update
2020 Propose range of new LWD values 2021 Engage stakeholders to communicate changes 2022 Finalize proposed LWD values 2025 Implement changes to coincide with the release of the IGLD update
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GREAT LAKES/SEAWAY REVIEW October-December, 2021
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