Dmitry Shteyn — Predicting Seiches and Water Level Shifts on the Great Lakes

The Myth of Great Lakes Tides

When new students join me for a weekend on Lake Michigan, one of the first questions I get is about the tide. It’s a logical question—when you're standing on the pier in Milwaukee looking out at an endless horizon, it feels like the ocean. The short answer is that the Great Lakes do have tides, but they are practically invisible. According to NOAA, the largest 'true' lunar tide on the Great Lakes is less than two inches. For all intents and purposes, we sail in a tideless environment.

However, if you’ve ever left your boat at a wall in Door County or tied up in a narrow river mouth like the Kinnickinnic, you know the water level is anything but static. What we experience instead are seiches and meteotsunamis, and understanding them is critical for both safety and performance.

What is a Seiche?

A seiche (pronounced 'saysh') is essentially a standing wave in an enclosed or partially enclosed body of water. Think of it like sloshing water back and forth in a bathtub. On Lake Michigan, this usually happens when strong, sustained winds blow from one direction for a long period, literally pushing the water to one end of the lake.

When the wind dies down or shifts, that water rushes back to the other side to find equilibrium. This oscillation can cause water levels to rise or fall by several feet in a matter of hours. For us sailors, this means the depth at your slip might change enough to ground your keel, or the current in a harbor entrance might suddenly become a four-knot torrent.

The Pressure Factor: Meteotsunamis

While a seiche is wind-driven, a meteotsunami is driven by rapid changes in atmospheric pressure, often associated with the fast-moving squall lines we see in the heat of a Wisconsin August. A sudden drop in pressure allows the lake surface to rise slightly, and as that pressure system moves across the water, it creates a wave that moves at the same speed as the storm.

I remember a race off the coast of Sheboygan where a heavy line of thunderstorms passed to our north. Even though we didn't get the wind, the water level at the yacht club dropped eighteen inches in twenty minutes, then surged back up just as quickly. If you aren't prepared with enough slack in your dock lines, a surge like that can pull cleats right out of the deck.

Practical Implications for Racing and Cruising

How does this affect your day-to-day sailing?

1. River Currents: In places like the Chicago River or the Milwaukee River, a seiche can actually reverse the flow of the river. If you are trying to motor out to the racecourse against a seiche-driven current, you might find yourself moving at half your expected speed.
2. Docking Safety: Always leave enough slack in your lines to account for a two-foot vertical shift. If you tie your boat tight to a fixed pier during a high-water surge, the boat may hang by its cleats when the water recedes, causing structural damage.
3. Navigation and Depth: On the Great Lakes, charts are based on Low Water Datum (LWD). During a negative seiche, the actual depth can be significantly lower than what your chartplotter says. If you're navigating a tight channel like the Death’s Door passage, that extra foot of water you thought you had might suddenly vanish.

The Rules and Safety

From a racing perspective, there isn't a specific rule in the Racing Rules of Sailing (RRS) that governs seiches, but Rule 1.1 (Helping Those in Danger) is always relevant. A major seiche or meteotsunami can create hazardous conditions in a harbor faster than a storm can.

If you see the water rapidly receding from a beach or a pier—a phenomenon often called a 'drawback'—get your boat away from the shallow water immediately. A massive surge is likely following it. It’s not a tide; it’s a localized displacement of thousands of tons of water, and it commands respect.