WES WEEK THREE: 24-28 September 2001

• Earth diary - a summary of the week's major environmental events on Planet Earth. [CNN]

Concept of the Week: Great Lakes Water Levels

In recent years the levels of Lakes Michigan, Huron, and Superior have dropped to near historic lows in response to weather conditions in their drainage basins. The drop in lake levels has had a serious impact on lake-based activities.

In the Fall of 2000, the Western Great Lakes were at their lowest levels in 35 years—the culmination of the greatest three-year drop in water level since detailed records began in the 1860s. Lakes Michigan and Huron water levels dropped some 100 cm (3.3 ft) over the prior three years. In early December 2000, Lake Superior was at its lowest level for that time of year since 1925 and only 16 cm (6.3 in.) above its historical record low for early winter. At the same time, Lakes Michigan and Huron were at their lowest early winter levels since 1964, some 22 cm (8.7 in.) above the 20^th century record low for the date. By contrast, in spite of reduced inflow from the upper lakes, early December 2000 levels of Lakes Erie and Ontario levels were only 10 cm (4 in.) below their long-term early winter averages--thanks in large measure to above average annual precipitation in their drainage basins during 2000. Lake levels continue to run low as of early September 2001.

For a plot of the historical record of lake-level fluctuations of Lakes Michigan and Huron since 1860, click on http://www.glerl.noaa.gov/data/now/wlevels/lowlevels/plot/5014.pdf (Adobe Acrobat required). The total range of lake-level variation during the 20^th century was about 1.9 m (6.2 ft).

Low lake levels have many economic, societal, and environmental consequences. Shallower than usual water requires expensive and environmentally damaging dredging to keep shipping channels navigable and ports open. Even with more dredging some channels cannot accommodate the draft of heavily laden freighters. Reduced carrying capacity impedes transport of grain and ore to processing facilities and markets. The Great Lakes supply most of the potable water for communities and cooling water for coal-fired and nuclear electric power plants located along their shores. A drop in water level may require costly repositioning of intake pipes. On the positive side, lower lake levels translate into broader beaches and wetland habitats and a respite from shoreline erosion.

The drop in levels of the Western Great Lakes is a response to weather conditions in their drainage basins. Rainfall, snowfall, and air temperature during winter and spring govern water levels of the Great Lakes in spring and summer. Spring melting of the winter snow pack in the drainage basin is particularly important and air temperature ultimately governs evaporation rates. A succession of mild winters in the late 1990s produced less than the usual snow pack in spring reducing the amount of spring runoff. And higher than usual temperatures increased evaporation of lake-water and streams draining into the lakes.

Concept of the Week: Questions

1. With rising temperatures, the rate of evaporation of water [(increases)(decreases)].
2. A trend toward snowier and colder winters and springs is likely to cause levels of the Great Lakes to [(rise)(fall)].

Historical Events

• 25 September 1939...A west coast hurricane moved onshore south of Los Angeles bringing unprecedented rains along the southern coast of California. Nearly five and a half inches of rain drenched Los Angeles during a 24 hour period. The hurricane caused two million dollars damage, mostly to structures along the coast and to crops, and claimed 45 lives at sea. "El Cordonazo" produced 5.66 inches of rain at Los Angeles and 11.6 inches of rain at Mount Wilson, both records for the month of September. (David Ludlum) (The Weather Channel)
• 25 September 1997...While much of San Bernardino and Inyo Counties in California are desert, between 1 to 2 inches of rain from former Pacific Hurricane Nora fell across a large portion of these counties. State highway 190 was washed out between Panamint Springs and Stove Pipe in Death Valley National Park. Flooding also shut down many other park roads. (Accord Weather Calendar)
• 25-26 September 1962...Torrential rains from severe thunderstorms over the hilly regions to the west and north of Barcelona Spain resulted in flooding, resulting in much damage. Many buildings collapsed. Tarassa was particularly hard hit with approximately 500 deaths. (Accord Weather Calendar)
• 26-27 September 1997...Thirty hours of rain that totaled between 8 to 14 inches fell across Pinellas and Hillsborough Counties in Florida, resulting in 5.3 million dollars in damage. In St. Petersburg, a 23 year old woman survived being swept 12 blocks in an underground drain before being dumped into a retention pond. (Accord Weather Calendar)
• 28 September 1986...Flash flooding swept a car into a swollen creek from off the Kamehameha Highway on the Hawaiian island of Oahu. The driver held onto some branches until help arrived, but the strong current stripped him of his clothes and watch. (Accord Weather Calendar)
• 28 September 1986...Torrential rains and floods were responsible for South Africa's worst natural disaster in Natal. As much as 35 inches of rain fell between the 25th and 29th to the northeast of Empangen, resulting in 317 deaths and 163 missing persons. Topsoil from some farms was completely washed away leaving only bedrock. (Accord Weather Calendar)

Return to WES Homepage

URL: WES:/news.html
Prepared by AMS WES Central Staff and Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu
© Copyright, 2001, The American Meteorological Society.