What is a seiche?

How a seiche works

NOAA explains a seiche as a standing wave: picture water rocking like a seesaw, with the biggest rise and fall at each end of the lake and very little movement at the middle, or node. Strong winds and fast changes in atmospheric pressure push water toward one end; when the wind drops, the water rebounds and keeps oscillating for hours or even days. On the Great Lakes the time between the high and the low can be four to seven hours^[1], close enough to the rhythm of an ocean tide that seiches are often mistaken for tides.^[1] NOAA distinguishes seiches from meteotsunamis: seiches are standing waves with periods usually longer than three hours, mostly confined to enclosed basins such as Lake Erie, while meteotsunamis are traveling waves with periods of minutes to two hours.^[1]

Why Lake Erie gets them

Lake Erie is long, shallow and lined up with the prevailing winds, so a gale blowing down its length can pile water up at the far end. In 1911 the Encyclopaedia Britannica described westerly gales lowering the water at the west end by 7 to 8 ft^[2] while raising it at the east end by 5 to 8 ft^[2].^[2] NOAA's bathymetry study describes the same effect from below: strong west-to-east winds raise levels in the eastern lake, and a compensating return flow at depth runs back west through the Pennsylvania Channel once the wind relaxes.^[3]

Seiches in history

NOAA cites an 1844 seiche of 22 ft^[1] that breached a 14 ft^[1] sea wall and killed 78^[1] people, and notes that as recently as 2008 strong winds raised waves of 12 to 16 ft^[1] on Lake Erie, with flooding near Buffalo, New York.^[1] Because a seiche can change water levels by several feet in a few hours, harbors at both ends of the lake can see unusual highs and lows during and after storms.

Questions people ask

Is a seiche the same as a tide?

No. A seiche is driven by wind and air pressure, not by the moon, but on the Great Lakes the time between its high and low can be four to seven hours, so it is often mistaken for a tide.

Why does Lake Erie have seiches?

It is long and shallow and lies along the path of strong westerly winds, which can pile water up at the east end. NOAA notes that Lake Erie is known for seiches.