Pre-drainage Relation of Lake Okeechobee to the Everglades
Christopher McVoy (Everglades Division) and Winifred Park Said (Hydrologic
Systems Modeling Division), South Florida Water Management District, West
Palm Beach, FL
Previously available descriptions of the hydrological connection between Lake
Okeechobee and the Everglades suggested that when the lake reached high stages
it would overflow by passing over an elevated custard apple rim. Our recent
research suggests that this description needs refinement and that the revisions
have significance for understanding water and nutrient balances of the predrainage Everglades.
Initial clearing of the custard apple swamp forest that originally bordered the
southern portion of Lake Okeechobee for agriculture began around 1900. As a
result, working estimates of its pre-drainage extent necessarily have been based
on two vegetation maps: Davis (1943) and Harshberger (1914). At the time of
Harshberger’s observations, patches of the original forest were still present, but
Harshberger’s mapping was casual and has proven inconsistent with detailed land
surveys conducted 1914-1916. Davis’ map was generally more detailed, but he
acknowledged having had to estimate the original custard apple extent, as
essentially none of it was left by the 1940s.
We used a combination of historical sources--land surveys commissioned by the
Trustees of the Internal Improvement Fund (State of Florida) in 1914-1916,
General Land Office surveys (federal) made in 1860-1907, an 1892 agricultural
drainage map, 1915 and 1948 soil maps, along with pre-drainage and early postdrainage shoreline descriptions--to create a pre-drainage map of the southern
shore of Lake Okeechobee where it bordered the Everglades.
Of the 47 miles of shoreline from the vicinity of Fisheating Creek on the west to
the former Pelican Lake on the east, the eastern 30 miles were custard apple
swamp forest. Based on the 1914-1916 land surveys, the width of the custard
apple swamp forest was only about 1 to 2 miles; considerably narrower than the 3
to 4 mile width estimated by Davis in 1943. The western 17 miles of the shoreline
was formed by sawgrass directly bordering the lake. Along this portion, a level
peat surface came up to the edge of the lake; the water surface extended smoothly
from sawgrass marsh into the lake. No intervening band of forest or mineral soil
was present between the lake and the Everglades in this area.
In the custard apple portion of the shoreline it is possible that the ground was
slightly elevated above the ground surface of the Sawgrass Plains south
(downstream) from the custard apple. However, there is also evidence suggesting
that this apparent rim may be an artifact of drainage. An early drainage engineer
noted that post-drainage subsidence was greater in the sawgrass rather than in the
custard apple soil, due to the much greater mineral fraction in the custard apple
soil. Available historical accounts are not precise about the relative pre-drainage
elevations of the sawgrass versus the custard apple portions of the shoreline.
Descriptions of the full southern shoreline overflowing at the same time would
suggest that the elevation of sawgrass and custard apple portions may have been
more similar than not. As peat soils, both were likely in equilibrium with
longterm average lake stages.
The presence of an extensive length of shoreline where sawgrass directly
bordered the lake significantly clarifies the relation of pre-drainage lake stages
relative to ground surface of the Everglades. The known water depth requirements
of a sustained sawgrass stand constrain the possible longterm average lake stages.
For example, lake stages could not have been consistently two to three feet above
(Everglades) ground surface, because this would have converted the stand into
slough-like vegetation. Similarly, the lake surface cannot have dropped below
ground surface for long portions of each year, because this would have caused
invasions of weedy and then woody species, e.g., dog fennel (Eupatorium
capillafolium), careless weed (Amaranthus australis), willow, elderberry, etc.
The presence of sawgrass at the shoreline suggests that lake stages typically rose
to 1.5 to 2 feet (45-60 cm) above the ground surface, and typically fell to 0.5,
perhaps as much as 1 foot (15-30 cm) below ground surface. Non-zero lake
outflows would have continued for eight to ten months during a typical year.
The presence of sawgrass directly bordering Lake Okeechobee also indicates that,
under pre-drainage conditions, persistance of the Sawgrass Plains was not
dependant on separation from the lake by an intervening band of custard apple
forest.
Christopher McVoy, Everglades Division, South Florida Water Management
District, 3306 Gun Club Rd, West Palm Beach, FL 33406. (561) 682-6510.
cmcvoy@sfwmd.gov