The Determination of Minimum Flows for Sulphur Springs, Tampa

DRAFT
It is possible that infrequent or slight salinity incursions could occur without impacting
the salt-sensitive species that have become reestablished in the spring run. It is difficult
however, to evaluate the frequency, duration and magnitude of salinity incursions that
such species could tolerate. Although there is a general knowledge of the ecological
requirements of these species, there is not literature that gives specific threshold salinity
values or durations of salinity that cause reductions in growth or reproduction. What is
known is that as flow from the spring pool goes lower, the likelihood for greater salinity
incursions will rise. Given this situation, breakpoints in the flow-salinity relationship can
be used to evaluate flows that reduce the likelihood of salinity incursions.
The data presented in this report indicate a clear breakpoint in the data for the upper
spring run at 18 cfs. This rate of flow, however, will not entirely preclude the possibility
of salinity incursions, as very high tides could cause brief salinity incursions into the
upper spring run. If a greater probability of salinity incursions could be tolerated, a
breakpoint of 13 cfs could be used. However, the data indicate that salinity incursions
into the upper river at 13 cfs could occur about half the days, with incursions occurring
on several successive days depending on tides in the river. Although the duration of
these incursions would be on the order of a few to several hours, there appears to be a
marked difference in the probability and severity of salinity incursions between 13 and
18 cfs. Given the unique character of the habitat and fauna in the Sulphur Spring Run,
the more conservative protection provided by an 18 cfs minimum flow is warranted for
routine application. Circumstances under which a smaller minimum flow could be
applied are described in the following sections.
5.2.4.1 Tide stage effects on salinity incursions
The height of tide stages in the Lower Hillsborough River can be important to the
magnitude and length of salinity incursions, as higher tides tend to push more brackish
water into the spring run during low flows. Water levels are recorded at continuous
recorders in the spring run and approximately 100 yards upstream in the river at the
Hillsborough River at Sulphur Springs gage. The stage records in the spring run date
from May 25, 1999 to present, while stage records in the river began on October 1,
2000. Although the period of stage records for the river is somewhat shorter, those
records are valuable for examining the effects of tide stage in the river on salinity
incursions in the spring run, as concurrent data are available for many of the
intermediate flows that occurred during 2001 and 2002.
As described in section 3.3, water levels in the river and spring run track each other
closely at higher tide stages. However, water levels in the river drop below water levels
in the spring run at low tide stages (Figures 3-2 and 3-4). This difference in low water
levels is due to the effect of the weir, which acts to retain water in the upper spring run
as water levels in the river fall. As water levels in the river rise, brackish water can be
pushed upstream of the weir if there is not sufficient flow to keep the incursions from
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