Cofrin Center for Biodiversity

Daily Changes in Green Bay Water

The water level in Green Bay fluctuates every day in a pattern similar to that of tides in the ocean. This process is known as a "seiche" and although it is similar to the tide of the ocean, it is not primarily due to the movement of the moon as are the tides, but rather to the effects of the prevailing winds and the shape of the Green Bay basin.

The seiche is highly variable, but consists of approximately 11 hour cycles (from high to low and back to high again, for example). The magnitude of the seiche also varies depending primarily on the strength, duration, and timing of the winds and on which part of the bay the measurement is taken. In lower Green Bay the average seiche is about 6-12 inches. An example of daily variations on Green Bay is shown below.

As the winds blow over the surface of the bay the water is pushed to the far end of the basin. If the wind stops or reverses (as it may when strong pressure systems move across the area) the water that has been pushed to the far end flows back toward the other end, and so on. In the absence of significant wind, the water may continue to move back and forth in the basin much as water in a shallow pan will continue to move back and forth after the pan has been moved and then placed at rest.

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