Rare Phenomenon Temporarily ‘Drained’ Lake Superior Over The Weekend – What Caused It?

On Saturday afternoon, after I was done with my Grandma's Marathon duties in Canal Park, I happened to notice that the mouth of the Duluth Harbor into Lake Superior looked...weird.

Just out past the piers, there were some pretty large waves pouring out into the lake from the harbor. So big, there were whitecaps on them. But the rest of the lake was fairly calm.

Here's a look at what I saw. The waves were easily 3-4 feet tall, but only out from the mouth of the canal.

The waves were concentrated out from the piers, and my first reaction was that it might have been from the rain the Duluth area got from the thunderstorms overnight that had swollen the St. Louis River, and it was pouring out into the lake.

It turns out it was related to the storms from overnight, but the cause was rarer and more interesting.

An "impressive seiche" rocked Lake Superior on Saturday

In the aftermath of the strong thunderstorms that swept across the Northland and Lake Superior early Saturday morning (June 21), a phenomenon called a "seiche" literally rocked the big lake.

READ MORE: Minnesota campground on Lake Superior named among the best places to camp in the country

While you don't generally think of Lake Superior as having tides like the oceans (NOAA considers it a non-tidal body of water), a seiche is one way this massive body of water experiences something like tides going out and coming in on the Great Lake. They happen from time to time, but Saturday's was more impressive than most.

A video captured north of Duluth, quite a ways north in Ontario, showed what looked like a scene you'd see on the ocean coast when the tide goes out. Receded water, and even boats left sitting on land where there once was water, were part of the awe-inspiring visual.

It's kind of like someone pulled the plug on a bathtub, isn't it? The water did eventually return. Here's a look at when the water returned in a second video.

Someone spending time on Isle Royale in Lake Superior also captured some footage from the weekend that included several sights from the island, including a time-lapse of the water going out and coming in on the shoreline.

The National Weather Service reported that the effects of this seiche were seen in several places on Lake Superior.

In a social media post on Sunday, they referred to this seiche as "impressive", reporting that those waves I saw coming out of the Duluth Canal were driven by an outflow of water from the harbor at an impressive rate of currents up to 6 feet per second.

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In Ashland, Wisconsin, at the breakwater, water levels changed by almost 4 feet up and down through the day on Saturday, with effects still felt on Sunday.

What causes a seiche?

NOAA explains that seiches are generally caused by strong winds, like those seen with the storms early Saturday, or rapid changes in atmospheric pressure. These can push water from one end of the lake to the other, causing a surge on one end of the lake and a drop in water levels on the other end.

The water eventually rushes back and can "slosh" back and forth for a while, like what happened over the weekend, eventually calming back down to normal.

How big can seiches get on the Great Lakes?

NOAA says Lake Erie is particularly known for seiches, and records of a 22-foot seiche breaching a 14-foot-tall seawall in 1844 paint a picture of just how powerful these can be.

One documented on Lake Superior in 1935 saw what was described as “a wall of water approximately 5 feet in height rose in Chequamegon Bay, endangering swimmers on the beach and flooding U. S. Highway No. 13 with approximately 3 feet of water” in the Ashland area.

Thankfully, they're not all that big or aggressive, but the one that happened on Saturday was certainly bigger than most that have happened in recent years.