Why Great Lakes Water Levels Rise and Fall

Stand on a Lake Michigan beach two summers apart and you can find yourself standing in two different places: the waterline migrates. Docks that were high and dry become wet; beaches that were broad become narrow, then broad again. Great Lakes water levels move, they have always moved, and the reasons are worth understanding because they shape everything from shoreline property to shipping to municipal water intakes.

A budget, not a bathtub

The clearest way to think about lake levels is as a running budget. Water comes in from precipitation falling on the lakes, from runoff out of the surrounding basin, and from the lake upstream. Water goes out through evaporation, through outflow to the next lake downstream, and ultimately out the St. Lawrence River to the ocean. When the budget runs a surplus for a few seasons, levels climb. When it runs a deficit, they fall.

The system is enormous. The five lakes together hold about a fifth of the fresh surface water on the planet, and they behave with the patience that size implies. Levels do not spike and crash like a river gauge after a storm. They drift, seasonally and across years, and the drift carries a lot of momentum.

The rhythms inside the drift

There is a reliable seasonal cycle. Levels are typically lowest in winter, rise through spring as snowmelt and rain feed the basin, peak in summer, and decline through fall as evaporation ramps up. Evaporation is at its strongest not in July but in late fall and early winter, when cold dry air moves over water that is still relatively warm. Ice cover matters for the same reason: a lake that freezes over early evaporates less.

Layered on top of the seasonal cycle are multi-year swings driven by stretches of wet or dry weather across the basin. These are the swings that make headlines, because a run of wet years can push lakes to the top of their historical range, eating beaches and undermining bluffs, while a run of dry years exposes shoals, strands docks, and forces freighters to load lighter. Both extremes have occurred within recent memory, sometimes within the same decade, which is itself the point: the record is a band, not a line, and the lakes use the whole band.

Weather also produces short-lived local drama. Strong winds and pressure changes can pile water up at one end of a lake and drop it at the other, a phenomenon called a seiche, which can move the local waterline dramatically for hours without changing how much water the lake actually holds.

Who keeps the books

Lake levels are among the better-measured things in American public life. The U.S. Army Corps of Engineers and NOAA, working with Canadian counterparts, maintain long-running gauge networks, publish current levels against historical averages, and issue seasonal forecasts. Regulation, in the engineering sense, exists only at the margins: outflows from Lake Superior and from Lake Ontario are managed at control structures under international agreements. The middle lakes rise and fall largely as the weather dictates. No valve exists that could simply hold Lake Michigan at a preferred height.

Why it matters inland

Even readers who never see the shore have a stake in the levels question. Dozens of Midwest cities draw drinking water through intakes fixed in the lakebed, engineered around assumed water depths. Harbors and the shipping economy that moves grain and ore calibrate to available draft. Shoreline communities zone, armor, and insure against a waterline that will not sit still. And the long record of fluctuation is a standing argument for humility in anything built at the water's edge.

The practical wisdom, repeated by coastal engineers and old dock hands alike, is to treat the historical range as the design condition. The lakes are not misbehaving when they leave the average. The average is just the middle of where they live.