The Immense Scale of the Laurentian Great Lakes
The Laurentian Great Lakes — Superior, Michigan, Huron, Erie, and Ontario — hold 21% of the world's surface fresh water. Unlike man-made inland reservoirs whose levels are manipulated daily by dam gates, Great Lakes water levels are governed by vast, multi-year natural climate cycles spanning thousands of square miles across the United States and Canada.
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1. Multi-Decadal Water Level Cycles
Over the past century, Great Lakes water levels have exhibited dramatic, cyclical fluctuations:
- In 2013, Lakes Michigan and Huron dropped to record all-time lows, exposing hundreds of feet of shoreline and grounding cargo freighters.
- Just seven years later in 2020, sustained heavy precipitation across the Upper Midwest pushed Lakes Michigan, Huron, and Erie to all-time record highs, washing away shoreline bluffs, submerging public beaches, and destroying coastal roads.
These cycles reflect multi-year swings in net basin supply (precipitation, lake surface evaporation, and watershed runoff).
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2. Understanding the IGLD85 Datum
Water levels on the Great Lakes are not measured in NGVD29 or NAVD88. Instead, they reference the International Great Lakes Datum of 1985 (IGLD85).
Because the Earth's crust is still slowly rebounding upward following the retreat of the glaciers (isostatic rebound), the northern shores of the Great Lakes are rising faster than the southern shores. IGLD85 accounts for this vertical movement to provide an accurate, unified measurement network coordinated between the United States (NOAA) and Canada (CHS).
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3. Wind Seiches: When Lakes Slosh Like Bathtubs
On shallow, elongated basins like Lake Erie, strong sustained winds blowing down the axis of the lake create powerful seiches:
- Water is physically pushed from the western basin (near Toledo) toward the eastern basin (near Buffalo).
- A severe seiche can raise water levels at Buffalo by 7 to 10 feet in a matter of hours while dropping levels at Toledo by 7 feet, stranding commercial freighters on mudflats.