The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
3.9 Relationships of Sulphur Springs discharge to the hydrology, water quality
and ecology of the Lower Hillsborough River.
The flow from Sulphur Springs has important hydrologic and ecological functions in the
Lower Hillsborough River system. During much of the year, Sulphur Springs provides
much of the combined inflow of fresh or low salinity water to the lower river. This inflow
is very important for establishing salinity distributions in the river, and during winter
months, maintaining thermal refugia for manatees and cold sensitive fish species such
as snook. The hydrology of the spring in relation to the river is described first below,
followed by a discussion of the physical-chemical and biological effects of the springflow
to the river system.
3.9.1 Hydrology of the Lower Hillsborough River
The watershed draining to the Lower Hillsborough River has an area of approximately
650 square miles. Approximately 90 percent of this area drains to the Hillsborough River
Reservoir. The dam that creates the reservoir is located approximately 16.3 kilometers
(10 miles) upstream of the river mouth. Downstream of the dam there is about twenty
square miles of highly urbanized watershed that drains to the lower river via storm
sewers and drains. During rains events, considerable freshwater inflow is contributed to
the lower river from the sub-basin below the dam (HSW 1992). In the dry season, these
events can be infrequent, with very little inflow contributed below the dam for prolonged
periods of time.
The majority of the inflow to the lower river comes from discharges from the
Hillsborough River Reservoir. Daily streamflow records for discharges from the reservoir
date back to 1939. There has been a significant decreasing trend in stream flow for the
reservoir, as evidenced by a time series of average flows shown in Figure 3-8.
Increasing withdrawals from the reservoir by the City of Tampa has been a factor
contributing to these decreasing flows. Average yearly withdrawals from the reservoir
were less than 31 cfs in the late 1940s, increasing to an average of 105 cfs (68 mgd)
over the last several years.
Other factors affecting the decline in average annual flows from the reservoir include the
operation of the Tampa Bypass Canal (TBC). Since the mid-1980s, the Bypass Canal
has been periodically used to divert high flows away from the reservoir to prevent
flooding in the urban Tampa area. In addition, above the influence of the TBC, there
appears to be a declining trend in inflows to the reservoir system (SWFWMD 1999).
Long-term changes in rainfall and groundwater levels in the region may be contributing
to these reductions in flow.
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