FLORIDA GEOLOGICAL SURVEY


The lower 15 to 18 miles of the river passes through the City of
Tampa. The city water supply is a reservoir created by a dam in
the river 10.2 miles upstream from the mouth. Tampa is vulnerable
to damages from floods in the Hillsborough River because of
extensive development of property in the flood plain.

 STREAMFLOW

 A summary of streamflow data for the gaging station on
Withlacoochee-Hillsborough overflow (station 39) is shown in table
5. The flow-duration curve is shown in figure 10. No flow occurs
in the channel at this point about 65 per cent of the time.
 Crystal Springs flows into the Hillsborough River in southern
Pasco County near the Pasco-Hillsborough County line. The average
flow of Crystal Springs (station 31) is 62 cfs, ranging from 20.3
cfs to 147 cfs. Downstream from Crystal Springs the base flow of
Hillsborough River is well sustained. The flow of Hillsborough
River near Zephyrhills (station 33), which includes flow from
Blackwater Creek, is reported to be 71 cfs or more for 90 percent
of the time (Menke, 1961, p. 29).

 CHEMICAL CHARACTERISTICS OF SURFACE WATER

 The chemical characteristics of water of the Hillsborough River
upstream from Crystal Springs are similar to those of the
Withlacoochee River between Eva and Dade City although the
mineral content is somewhat higher.
 The water of the Hillsborough River at the Withlacoochee-
Hillsborough overflow contained calcium and bicarbonate as the
principal dissolved mineral constituents. The water contained
color that ranged from 80 to 150 units. The mineral content ranged
from 41 to 121 ppm.
 The water of Crystal Springs had a mineral content of about
 170 ppm, was clear, and contained calcium and bicarbonate as the
 principal dissolved minerals. The water was hard and alkaline.
 During the periods of low flow, the chemical characteristics of
the water of the Hillsborough River near Zephyrhills (below
Crystal Springs) are essentially the same as those of water of
Crystal Springs. Figure 17 shows the relation of the mineral
content to discharge. During periods of high flow the mineral
content of water is low. A more detailed discussion of the chemical
character of the water of the Hillsborough River is given in a
report by Menke (1961, p. 28-36).