In Florida, the level of a lake may be dependent upon many factors including weather patterns, surface soil types, nearby groundwater pumping, and hydrogeologic conditions, such as "perched" groundwater—water held in sands below the surface but typically separated from deeper groundwater by layers of clay, or even rock—and connection to deep aquifer sources (such as the Floridan aquifer).
When the weather is dry, lake levels are often low and with abundant rain, lake levels generally rise. This is due to dependence of many Florida lakes on lateral seepage from perched groundwater systems into the lake. Other lakes are highly dependent on actual rainfall and/or stormwater runoff. A smaller number of Florida lakes are "spring fed," meaning that their water supply is mostly from deep aquifer sources where water flows up and into the lake.
If you plan to build a lakefront home or a dock, it's a good idea to find out what the historic water levels are for a lake. Such information can be crucial to the success of your project. Failure to do so could result in unexpected flooding or the opposite; during drought conditions, some lakefront residents may be surprised to find their lake has suddenly shrunk out of sight.
Aside from weather patterns, there are a few other naturally occurring factors that can affect water levels. Geology can have a lot to do with a lake's ability to "hold" water. For instance, lakes with sandy or porous soils are more susceptible to losing water through seepage, whereas clay soils can act as a barrier and help lakes retain water. Evaporation is another major factor. For example, in central Florida, evaporation rates range from 25 inches to 50 inches per year!
Fluctuations in lake levels may also be the result of human activity such as water withdrawal for drinking and irrigation, channelization, dams, dredge and fill projects and shoreline development.
So how does one know whether a lake level is low, high or "normal"? Thanks to a legislative mandate (Section 373.042 FS), Florida's Water Management Districts are required to estimate minimum flows and levels (MFL): minimum water levels for lakes and minimum flows for streams.
With 7,800 lakes in the state, this will take some time, but the information will certainly be useful.
Much of the water level data are collected by individuals who live on lakes. They read and record weekly and/or monthly water levels by reading the graduations on staff gages deployed by scientists in the lake but near the shoreline (see the image above of a staff gage in Lake Helen). Today, automated systems that continuously measure water level are being deployed in many lakes.
Level measurements are reported in feet and reference mean sea level (MSL). For example, a lake level of 35.50 feet is 35 and 5/10 feet above mean sea level. Level measurement reference systems have been developed using vast elevation (level) data networks, such as NGVD (National Geodetic Vertical Datum) and more recently NAVD (North American Vertical Datum) General Adjustment (1988, 1991), and are used to ensure the accuracy and representative quality of level measurements throughout Florida and the world.
Water level standards (such as Minimum Flows and Levels) are reported with their geodetic reference; for example, if a minimum water level for lake has been determined to be 32.25 ft (MSL), it would be reported as 32.25ft (NGVD). For more information about Minimum Flows and Levels, and to determine whether or not level standards have been established for your lake of interest, visit the lake hydrology pages of your atlas.
While looking at the Water Levels section on this page, notice that four measurements are provided:
Latest Value: This provides the latest water level measurement from the lake, along with the sampling date. Both are represented in red lettering.
High Water Levels:
The FEMA 100-year flood is the elevation level that the Federal Emergency Management Agency defines as "having a one percent chance of being equaled or exceeded in any given year."
The NHWE (Normal High Water Elevation) Flood gives an indication of the expected high level of the water during normal hydrological conditions.
Historic Norm for Month: This is the average of all the data samples collected during the same month in which the latest water level measurement was taken. This gives an indication of how the current level compares to measurements taken at approximately the same time during previous years.
Historic Range: This is gives the minimum and maximum lake levels that have been recorded for the water body over time. This number is provided in red and below it you can find calendar dates for the entire period that it was sampled (e.g., 12/5/1974 through 9/3/2004).
Due to the high number of lakes that exist in this state, many of them have not yet been professionally surveyed and had a gage of some type installed. Water level data for water bodies with gaging stations may not always be accurate or available due to damage, or mechanical or electrical problems that cause instrumental errors. In some cases, water levels may even drop so low that an accurate measurement can not be made without professionally resurveying the water body first. Be sure to check for gaps in data that might result from gaging problems before using water level data.
Also, it is important to remember that one should never jump to conclusions when comparing lakes, even lakes that are in close proximity. As discussed earlier, there are many factors that can affect a lake's water level. Even minor differences in geology or the hydrology of the area can allow one lake to fill differently than a neighboring lake.
For more information about the water level measurement and data, and minimum and maximum estimated water levels for your lake, contact your local Water Management District.