Key West 



Key Largo 



Figure 18. Areal extent of tidal types and locations of stations with illustrated tidal curves. 



decreases in a southerly direction (Walcott, 1975). Thus, for a constant water volume in 

 Lake Michigan, the water level would be rising relative to the land in Cliicago and falling 

 relative to the land in the Straits of Mackinac. Moore (1939a) concluded that the elevation 

 of the Great Lakes above sea level were not known with any precision, and that sea level as a 

 datum for precise elevations in the Great Lakes region was of doubtful value. 



Ramey (1952) discussed the changes in levels of the Great Lakes, identifying changes due 

 to the construction of various navigation facilities and changes due to variable winds and 

 variable rainfall. Although not discussed by Ramey, the construction of storm sewers, 

 clearing of forests, and other actions wliich affect the speed with which rainfall reaches the 

 lakes, change the natural hydrologic cycles of the lakes and may also change the lake levels. 



An international coordination committee, representing tlie U.S. Army, Corps of 

 Engineers and the Departments of Transport, Mines, and Technical Surveys and Resources 

 and Development, Canada, was established to develop an identical set of hydraulic and 

 hydrologic data for the Great Lakes basin for use by both countries. This committee 

 adopted a Great Lakes datum called the International Great Lakes Datum (IGLD) of 1955 

 (Coordinating Committee on Great Lakes Basin Hydraulic and Hydrologic Data, 1961). This 

 committee is composed of representatives from tlie U.S. Army, Corps of Engineers, tlie 

 National Oceanic and Atmospheric Administration (NOAA), and the Department of the 

 Environment in Canada. It is the first datum for tlie Great Lakes region to gain wide 

 acceptance in both the United States and Canada. 



49 



