I live on the St Mary's River on the east end of Lake Superior. Seiches are not uncommon here, I notice them several times a year. I think they are mostly caused by different atmospheric conditions at different ends of the big lake. A low pressure area moves across the Duluth area raising the water level at that end of the lake and lowering it at our end. Then the water sloshes back and forth a couple of times before it settles back to normal. The biggest seiches I've seen are close to three feet-- That is, the water rises about three feet then drops about six feet before starting to come up again. From high water to low it takes about an hour. So I guess the period (high to high) here is about 2 hours or so. When a seiche sucks the water out we have an exposed sand and silt in front of our house going out over two hundred yards in what is normally thigh deep water. One time my two sons looked out to see some large carp flopping on the wet sand. They went out and carried all the carp to the deeper water where they let them go.

: I saw a good-sized on a couple of years ago. It filled in the mouth
: of a river by a couple of feet and then went out about 30
: minutes later. It had a really long period. Last year, we were
: in a harbor on the south shore of Lake Superior and watched one
: come in and go out several times. It dropped the water level in
: the harbor by 6 to 8 inches every time it went out. Took about
: 30 minutes between high and low points.

: I heard of one on Lake Superior that left boats in the Apostle
: hanging from their lines. In '54, on Lake Michigan, a 10-foot
: seiche killed eight fishermen. Crazy stuff!