REPORT OF INVESTIGATIONS NO. 42


moves generally from the southeastern part of the Green Swamp
area eastward toward the Kissimmee River Basin; westward toward
the Hillsborough and Withlacoochee River basins; southward
toward the Peace and Alafia River basins; and northward toward
the St. Johns River basin.
 Figures 35 and 36 show the shape of the piezometric surface of
the Floridan aquifer in the Green Swamp area and vicinity during
a wet period (November 1959) and during a dry period (May 1962).
Analysis of the maps shows the direction of movement of ground
water did not change appreciably from wet to dry periods but the
elevations of the piezometric surface declined. The decline was
greatest along the southern and western borders and least in the
interior of the Green Swamp area. Lows or troughs in the
piezometric surface indicate that ground water discharges into
Withlacoochee River through a spring at the mouth of Gator Creek
and downstream from Dade City; into Hillsborough River at Crystal
Springs; into Blackwater Creek; into Davenport and Reedy Creeks;
and into Horse Creek. Closed depressions, such as those in the
vicinity of Lakeland, indicate the effects of pumping.
 Natural hydraulic gradients, indicated by the spacing of the
contour lines in figures 35 and 36, are steep toward the Hillsborough
River on the western side of Green Swamp and toward Reedy,
Davenport, and Horse Creeks on the eastern side. The base flow
of Hillsborough River below Crystal Springs is sustained by more
than 50 cfs of ground-water inflow from the Floridan aquifer. The
base flows of streams on the eastern side of the area are sustained
by relatively small amounts of ground-water inflow from the
Floridan aquifer. Obviously then, the steep gradient toward the
east is caused by some factor other than a high rate of
ground-water discharge. The geology along the eastern side of the
Green Swamp area (see fig. 8, A-A') suggests that the steep
eastward gradient is due to a barrier, or constriction in the aquifer,
that was formed by natural grouting (sink-hole collapse and
cavity-fill) along the fractures and joints in the limestone. Thus,
the barrier effect decreases the ground-water outflow and a piezo-
metric high is formed along the eastern side.
 Figure 37 shows the decline in the piezometric surface from the
wet period (1959-60) to the dry period (1962). Water levels
declined least in the interior of the Green Swamp, in the
Hillsborough River basin, and in the Kissimmee River basin
(Davenport, Horse, and Reedy-creeks). Water levels declined most
along the southern (near Lakeland), western (near Dade City),