Middle St. Johns - Florida Department of Environmental Protection

Water Quality Assessment Report: Middle St. Johns
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may also be designated as “Special Waters” based on a finding that the
waters are of exceptional recreational or ecological significance, and are
identified as such in Rule 62-302, F.A.C.
Physiography, Soils, and Geologic Framework
The land surface of the St. Johns River has been shaped predominantly
by marine processes, with later modification by solution of the underlying
carbonate material. The St. Johns River Valley was formed during
the Pleistocene Epoch, during a period of barrier island formation. It is
believed to have formed as part of a regressional or progradational beach
ridge plain. Fluctuating sea levels created marine terraces at approximately
10, 30, 100, and 150 feet above the present sea level. Terraces are relatively
flat areas separated by bluffs. They tend to parallel the modern-day coastline
and are often associated with wind-built sand dunes. The course of
the river was probably determined in part by the location of swales between
relict beach ridges.
The St. Johns River in the middle portion of the basin, for the most
part, lies in what is called the St. Johns Offset. It is called an offset because
this part of the river has changed course from the valley of its origin to an
older, narrower, westerly course (Scott, 1979). East of the St. Johns Offset
and bounding it are the DeLand and Crescent City Ridges. Soils on these
ridges comprise medium-fine sand and silt and clayey sand (Scott, 1979).
To the west of the St. Johns Offset are the Marion Uplands and Mount
Dora Ridge. Clayey sand and medium-fine sand and silt underlie the
upland and ridge, respectively (Scott, 1979). Common characteristics of
the ridges are deep lakes, closed lake basins, low water tables, and subsurface
drainage (Knochenmus and Hughes, 1976). The uplands are
generally moderate in relief, with numerous closed lakes that may become
connected during high water, shallow lakes, and moderate depths to the
water table (Knochenmus and Hughes, 1976). About at the latitude of
Sanford, the St. Johns River returns to its course in the Eastern Valley. Soil
types under the St. Johns Offset and Eastern Valley are generally shelly
sands and clay (Scott, 1979).
Relict
Ancient shorelines.
Ground Water Resources
Aquifers
Aquifers in the Middle St. Johns Basin ground water flow system
include the upper and lower Floridan aquifer system (Floridan) and the
surficial aquifer system (Snell and Anderson, 1970; Spechler and Halford,
2001). The Floridan is the deepest aquifer system. The lower Floridan
includes the upper part of the Cedar Keys Formation of Paleocene age,
along with the Oldsmar Formation and the lower part of the Avon Park
Formation, both of Eocene age. The upper Floridan includes units of the
Hawthorn Group of Miocene age and some undifferentiated overlying