Fluctuating lake levels
During an extended dry period, a canoe and dock sit on land that had been lakeshore.
Thousands of large and small lakes dot the Florida landscape, adding to the state’s unique environment. Florida’s lakes add to the quality of life for the state’s residents and visitors, offering an abundance of opportunities for fishing, boating, wildlife viewing, swimming and other recreational activities. Lakes also provide habitat critical to the survival of alligators, turtles, fish and birds such as hawks, eagles, ducks and herons. The natural beauty and recreational opportunities provided by Florida’s lakes contribute significantly to the state and local economies.
Characteristics of the lakes
Florida’s lakes vary greatly in their size. The surface areas of the region’s lakes range from thousands of acres to less than one acre. Many Florida lakes were formed from sinkholes — areas where the erosion of underground limestone allowed depressions to form in the Earth.
Some lakes are naturally connected to other water bodies, allowing water and aquatic and wetland-dependent organisms to move among lakes and between lakes and rivers. Other lakes are artificially connected by canals dug many decades ago to relocate water, in many cases to protect farms and communities from floods.
Most of Florida’s lakes are landlocked, receiving water directly from rainfall, from storm water and from seepage from surrounding soils. Rainfall, either directly or indirectly from stormwater runoff and shallow groundwater seepage, is the single greatest factor affecting water levels in Florida’s lakes. In landlocked lakes, water is predominantly lost through evaporation and seepage from the lakes into the Floridan aquifer.
While Florida averages approximately 50 to 55 inches of rain annually, much of the water is lost to evaporation each year. In drier than normal years when the amount of evaporation and seepage exceeds the amount of rainfall, lake levels decline. In wetter than normal years, lake levels increase, leading some lakes to expand in size as water flows into adjacent wetlands and may flood to the uplands.
Fluctuating lake levels
Natural fluctuations in water bodies encourage vegetation germination and growth. This provides habitat for fish and wildlife, and helps improve water quality.
Water levels in Florida’s lakes vary over time from short-term, annual wet season rises and dry season declines to long-term fluctuations caused by multi-decade rainfall trends.
Lake levels can naturally change seasonally and over longer periods of time. During periods of below-normal rainfall, lake levels drop, at times becoming so low that boating access becomes limited. Lake levels may continue to decline for years, affecting the use of docks and affecting lakeside businesses. Lake levels generally rebound, leading to water levels so high that lakeside property owners may, at times, experience too much water.
While the St. Johns River Water Management District has some control over lake levels in lakes such as the Harris Chain of Lakes in Lake County and Lake Washington in Brevard County through the operation of dams or other water control structures, water level fluctuations in most lakes are predominantly driven by rainfall. In lakes without water control structures, such as is the case with most of Florida’s lakes, little can be done to artificially raise or lower water levels.
Water withdrawals from surface and groundwater sources can also affect lake levels. As water levels belowground decline, the difference in elevation between the lake water surface and the aquifer water surface increases, causing an increase in water seeping from lakes into the aquifer. The result may be fewer flooding events and more dewatering events over a long period of time. The district’s minimum flows and levels (MFLs) program typically establishes multiple MFLs that may allow for some consumptive use of water while protecting aquatic and wetland systems from significant harm by requiring a minimum number of flooding events and allowing for a maximum number of dewatering events over a long period of time.
Effects of fluctuating lake levels
While the low water levels and exposed lake bottoms that occur during dry conditions are unattractive to many and can interfere with commercial and recreational activities, fluctuating water levels are normal and beneficial for Florida’s lakes.
The fluctuations that bring extremely low water levels benefit the entire ecosystem. Benefits include:
- Growth of new vegetation on exposed lake bottom and in adjacent marshes due to seed germination on dewatered soils
- Improved habitat for fish and other aquatic organisms, and improved water quality (upon reflooding) as a result of increased vegetation growth during dewatering events
The fluctuations that bring extremely high water levels also provide environmental benefits, including:
- Rehydration of adjacent wetland vegetation and soils
- Improved wildlife habitat through plant growth and soil hydration
- Increased fish population through provision of habitat for fish feeding and spawning
- Improved water quality through the settling and removal of pollutants in wetlands
Additional information about fluctuating lake levels is available in “Hydrology of Central Florida Lakes: A Primer,” published by the U.S. Geological Survey in cooperation with the St. Johns and South Florida water management districts.
Updated on 11-2-2010