The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
February 2002 when there was no flow from the dam. However, water temperatures at
all the stations were similar in December 2002, when flow from the dam averaged 210
cfs. As with the EPCHC sites, water temperatures in the river during summer months
are highest near at the mouth of the river at Platt Street.
Time series plots of average daily values for surface and bottom temperatures for the
period 1990 - 2002 at the four USGS recorders are shown in Figure 3-31. The period of
record is longest at the Rowlett and Nebraska Avenue stations, as data collection at the
Platt and Hanna's stations began in the first part of 2001. Data were not available after
September 2002 for Rowlett at the time of this report. Water temperatures went below
20 o C at all recorders in the river every year. With the exception of the Nebraska Avenue
site, water temperature dipped below 15 o C every year. The winter of 2002-2003
appeared to be the coldest, as water temperatures at Platt and Hanna were below 15 o
C for almost two months. Winter water temperatures were somewhat warmer at
Nebraska Ave., as temperatures dipped to 15 o C only twice in the four years of record.
3.11.3 Vertical and diurnal variations in river water temperatures
Differences between surface and bottom temperatures for these same four recorders
are plotted in Figures 3-32. The largest differences between surface and bottom
temperatures appear to be near the spring at I-275, followed by Platt. At both stations,
there are fluctuations when the surface is either warmer or cooler than the bottom, likely
reflecting the effect of short-term meteorological conditions on surface waters. Surface
to bottom differences were generally smaller at the Hanna's and Rowlett recorders,
which are closer to the dam.
In addition to the spatial, seasonal, and vertical temperature variations, there are
significant thermal differences that occur at shorter time scales. Figure 3-33 illustrates
diurnal variations in water temperature, expressed as daily ranges in surface and
bottom temperatures. Bottom temperatures are generally more stable than surface
temperatures, where daily variations in air temperatures have a greater effect. Diurnal
swings in surface water temperatures were often in the range of 2-3 o C and reached as
high as 5-7 o C at I-275 on some days. Diurnal temperature variations appeared to be
somewhat greater in the upper river, compared to the mouth of the river at Platt St.
3.11.4 Temperature variability and relation to biological use
The results presented above show there are both horizontal and vertical gradients in
water temperature in the lower Hillsborough River and these gradients change
temporally on short-term and seasonal time scales. Highly motile organisms such as
fishes and manatees can migrate in the river to seek suitable temperature regimes in
times of temperature extremes. Due to its discharge of relatively stable water
throughout the year, Sulphur Springs exerts a clear effect on the thermal characteristics
of the lower river, particularly in its upper reaches and during times of no flow from the
dam.
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