REPORT OF INVESTIGATIONS No. 42


saturation should be lowest in recharge areas, and should increase
as water moves through the aquifer.
 It would appear then that factors other than high recharge
rates contribute to the causes of the Polk high. It can be inferred
from geologic and hydrologic data that a relation exists between
the water movement and structural deformation in the Green
Swamp area. This is indicated at the surface by the parallel
linearity of ridges and surface drainage systems, and in the
subsurface by the presence of an anticline and related faults. Also
of importance is the relation between structural deformation and
subsequent solutional deformation indicated by the parallel linearity
of sinkhole lakes.
 High piezometric levels in the southeastern part of the Green
Swamp area are believed to be the result of a relatively slow rate
of ground-water outflow which is probably caused by sand-filled
fractures, caverns, and sinkholes. These act as a natural grout
which decreases the transmissibility of the aquifer. Although the
coefficient of transmissibility of the Floridan aquifer is generally
high, it is variable, ranging from about 200,000 to 1,200,000 gpd/ft
(table 11). The lower value applies to the eastern part of the Green
Swamp area where the piezometric high exists.
 Flood-control and conservation reservoirs proposed for the
 Green Swamp area would provide a partial solution to the flood
 problems in the lower Hillsborough River and lower Withlacoochee
 River basins. Total impoundment in the Green Swamp area of a
 flood equal to that of March 1960 would reduce the flood crest of
 the Hillsborough River at Tampa by about 1 foot. Impoundment
 of the March 1960 flood in reservoirs proposed for the Green Swamp
 area would have reduced the flood crest of the Withlacoochee River
 at the Trilby gaging station by about 4 feet and at the Croom
 gaging station by about 1.7 feet. Reservoirs in the Green Swamp
 area would be less effective in reducing flood stages further
 downstream on the Withlacoochee River.
 Impoundment of floodwaters. in the Green Swamp reservoirs
 would probably have little effect on net ground-water outflow from
 the Green Swamp area but would increase base flows downstream
 from the reservoirs.
 Impoundment of flood waters in the Southeastern Conservation
 Area would increase the rate of seepage to the Floridan aquifer.
 The rate of seepage would be more significant during dry periods
 when the piezometric surface is lowered by pumping in the
 developed areas south of the Green Swamp. Therefore, water


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