Lake Okeechobee ranks as the second largest freshwater lake in the United States after Lake Michigan. The lake is centrally located in South Florida. The lake originated by gentle epeirogenetic uplift of an irregular marine surface. Uneven sedimentation by currents formed the original submarine depression during the Pliocene. Recession of the sea during the Pleistocene probably formed the lake itself. The original lake was increased greatly by organic peat accumulation along the southern rim of the lake about 6,300 years ago. This natural dam reached its maximum elevation less than 2,000 years ago. In its natural state Lake Okeechobee had no channelized outflows. The 382 km2 emergent marsh represents a resource of regional significance that is heavily utilized by both feeding and nesting wading birds during the latter part of the dry season, when most other interior South Florida wetlands have dried. In years of below average rainfall, the lake marshes may provide critical habitat which supports nesting colonies from other locations.

Since the early 1900's major channel improvements and canal systems have been introduced. Most of the present drainage and water supply system was designed and constructed by the U.S. Army Corps of Engineers and is maintained and operated by the South Florida Water Management District. Erection of low muck levees along the south and southwest shores eliminated sheet flow into the Everglades in the early 1920's. These original levees were breached by the 1926 and 1928 hurricanes with the resultant loss of over 2,100 lives. This catastrophe resulted in the Corps of Engineers building a larger levee around the southern end of the lake. Construction of this section of levee was completed in 1937. Later, between 1960 and 1964, the levee was raised several additional feet and extended around the entire lake. Today the lake is completely surrounded by a large levee.

Lake Okeechobee contains highly mineralized water for an eastern lake which is dominated by calcium, sodium, chloride and bicarbonate ions. The lake also has moderately high and variable color levels, high daytime dissolved oxygen concentrations and an average percent O2 saturation of 100 percent. The geological formations in the lake's watershed are naturally rich in phosphorus and nitrogen; thus edaphic factors seem to provide the basis for the lake's eutrophic condition (8, 9).