Cofrin Center for Biodiversity

Extremely Low Water Levels on Green Bay
by Gary Fewless, Herbarium Curator
December 1, 1999

click on image for enlargement

Low water in Green Bay has been a prominent topic of discussion in northeastern Wisconsin in the Summer and Fall of 1999. A panoramic view of the lower bay shows extensive areas of newly exposed mud. These mud flats provide feeding opportunities for shorebirds and sites for colonization by wetland plants. Although the low water can be a problem for shoreline property owners, boaters, duck hunters and others, such fluctuations are natural to the ecosystem and contribute to biodiversity in Green Bay, especially in the coastal marshes.

The water level of Green Bay has a history of dramatic change. The U.S. Army Corps of Engineers has recorded water level for Green Bay since the late 1800's, and it is clear that water level varies on several time scales, including daily, annually, and on roughly ten to twenty year cycles. Since 1964 monthly mean water level of Green Bay has fluctuated over 6 feet!

Each year the water levels rise to a high in about July (the result of spring runoff of snow melt and substantial spring and early summer rains) and usually fall to a low in February or March. In addition to the annual changes, the water level can rise and fall over a period of years, in response to cumulative effects of climate.

The graph below plots monthly mean water level from 1962 to 1999. Data is from the U.S. Army Corps of Engineers.

Green Bay montly water levels.

Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects