The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
presented in this report. Dissolved oxygen is also measured at three continuous
recorders operated by Tampa Bay Water at Sligh Avenue, Columbus Avenue and the
Crosstown Expressway (kilometers 10.6, 3.7, and 0.2 respectively). However, the DO
data presented below are limited to vertical profile measurements included in the three
data sets first described, due to their large number of observations and good spatial
coverage in the area between the dam and Sulphur Springs.
DO concentrations in the Lower Hillsborough River are related to the amount of
freshwater inflow, particularly in the reach of the river above Sulphur Springs. Box and
whisker plots of surface and bottom DO data in segments of the river taken from the
combined Tampa Bay Water HBMP/HIMP data are presented in Figure 3-22 for flow
and no-flow conditions at the Hillsborough River dam. Different patterns are observed
between the upstream and downstream reaches of the lower river. Downstream of km
7, the distribution of surface DO values is generally higher for no-flow conditions.
Conversely, upstream of Sulphur Springs (km 12.9), surface DO values are higher for
flow conditions at the dam. A similar pattern holds for bottom DO values in the
HBMP/HIMP data - values for no-flow conditions are higher below km 7 and values for
flow conditions are higher above km 13. It should be noted that bottom DO
concentrations are lowest in the segment nearest to Sulphur Springs (km 12), which is
probably related to the steep density gradients that occur there during no-flow
conditions (Figures 3-13 and 3-16).
Only some of these patterns are apparent in the box and whisker plots for the EPCHC
stations (Figure 3-23), which for three of the stations represent a much longer period of
record (Figure 3-12). In contrast to the HBMP/HIMP data, distributions of surface and
bottom DO concentrations in the EPCHC data appear similar between flow and no-flow
conditions at the two downstream stations (km 0.0 and km 3.5). An important similarity
in the two data sets is they both show that surface and bottom DO concentrations
improve with flow in the reach of the river above Sulphur Springs (upstream of km 13).
At Sligh Avenue (km 10.6) the EPCHC data show a pattern similar to the HBMP in that
surface DO values are similar between flow and no-flow conditions, while bottom DO
concentrations are somewhat higher during flow conditions. These results reflect that
the water column is highly stratified in this reach of the river during no-flow conditions,
partly due to the effect of Sulphur Springs, which lowers salinity in the surface layers
(Figure 3-16). The improvement in DO during flow conditions is likely due to the
inclusion of high flow values in those data, which diminish vertical stratification at that
location (Figures 3-18 and 3-19).
3.9.4 Effects of low flows on salinity distributions and dissolved oxygen
concentrations between the dam and Sligh Avenue
The effects of freshwater inflow will be examined throughout the river as part of the reevaluation
of minimum flows for the Lower Hillsborough River. For the evaluation of
minimum flows for Sulphur Springs, special attention is given in this report to the reach
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