During the afternoon of May 30, 2011, a seiche developed on Lake Superior. This seiche caused water levels to rapidly fluctuate by several feet or more in just in a very short period of time (less than 2 hours). In fact, some parts of Lake Superior saw a 2 to 4 foot fluctuation of water levels in a matter of minutes (Whitefish Bay saw fluctuations of up to 6 feet in less than 2 hours).
A seiche is normally found on enclosed bodies of water such as the Great Lakes, and is simply a standing wave that oscillates, or "sloshes", back and forth. At least on the Great Lakes, seiches usually develop when rapid changes of wind and/or barometric pressure move across the lake, causing water levels to rise on one end of the lake and fall on the other. This type of situation can occur with strong autumn storm systems or with spring and summer thunderstorm complexes. The water then begins to slosh back and forth, sometimes for several hours or even several days. On May 30th, there was a dying thunderstorm complex that moved over the lake. This complex created a wake low over the lake, which helped to cause increased easterly winds on the lake. In turn, this allowed a seiche to develop. The graph below shows a plot of barometric pressure and water level at the Marquette NOS Water Level Site. Note the rapid pressure fall at around 1315 EDT (115 pm EDT) that corresponds to when the remnant thunderstorm activity moved over the site. This passage of the thunderstorm complex coincided with the start of the seiche.
Due to the relatively low water levels on Lake Superior in 2011, no significant problems were reported with this seiche. In fact, most seiches on Lake Superior tend not to be extreme. However, there have been a few seiches in the past that have caused some damage on Lake Superior. One such seiche in June 1939 caused extensive damage to docks and structures near Marquette. With that being said, if you see significant rapid lowering of water levels near the shoreline, it is possible that a seiche has formed, and it is possible that the water levels will rapidly rise in a short period of time. In this case, it would be wise to stay away from the beach until the water levels stabilize.