Alafia River Minimum Flows and Levels - Southwest Florida Water ...

gradually removed (this appears as more of a monotonic trend in the flow record) . This
explains the observation by Stoker et al. (1996; page 65), that at the Alafia at Lithia
gage, "Although annual-mean and annual high flows have decreased, the 7- and 30-day
low flows increased from about 1957 to 1966 and then decreased from about 1967 to
1992."
Stoker et al. (1996) also reported that "cumulative annual total rainfall at Plant City
[although outside the watershed, it is the closest long-term gage] was plotted against
the cumulative annual-mean discharge at site A-3 [Alafia at Lithia gage]. If the change in
streamflow characteristics was due only to a change in rainfall patterns, the resulting
[double mass] plot would be a straight line. . . The change in slope after about 1980
indicates a decrease in discharge relative to rainfall." Hickey (1998) also relied heavily
on double-mass plots in his analysis; however, he came to a different conclusion.
"Although Hickey (1998) concluded that climate was largely responsible for the
decreasing trend, he did note that at the Alafia River at Lithia stream flow decreased
about 44 cfs in the period between January 1962 to December 1981. He speculated
that these flow declines were the result of mining, [and] were related to a substantial
decrease in water being discharged rather than landscape changes that resulted in
hydrologic alterations" (Kelly et al. 2005, page 2-49). Our analysis of water quality data
validates Hickey's conclusion. The plot of dissolved fluoride in our document or Stocker
et al. (1996; Figure 21 page 31) shows a dramatic decline in concentration around
1980; the time by which we assert that mining related discharges were greatly curtailed.
It should be noted, while much more quantitative numbers may be desirable, Hickey
(1998) as quoted above did indicate that mining related discharges were on the order of
44 cfs (28 mgd) for the twenty year period extending through the 60's and 70's.
To clarify, we do maintain that substantial flow declines in the Alafia River are consistent
with a step change in climate related to the AMO. In addition, removal of phosphate
mining related discharges resulted in an apparent monotonic decreasing trend in flows
that is particularly evident under low flow conditions. The beginning and ending of this
flow augmentation period is clearly discernable in the historic flow and water quality
records for the Alafia at Lithia gage site.
Our comments regarding Attachment B (from Dr. Ann Hodgson, Florida Coastal Islands
Sanctuaries Program) are considerably shorter. As noted Dr. Hodgson's comments are
focused on riverflow and wetland dependent avian species or their forage. Firstly, the
Alafia River MFL is being developed in two parts, an upper freshwater segment (the
current subject) and an estuarine segment. Dr. Hodgson's concerns relative to the
movement of the salinity prism will be explicitly addressed as the MFL for the estuarine
segment is developed. There were, however, several comments with respect to our
PHABSIM related analyses, and isolated wetlands that we will attempt to address. We
explicitly recognize the importance of floodplain wetlands to the ecology of the river
system (pages 3-6 to 3-7 of our report) and have made a concerted effort to address the
maintenance of these systems. In the short term, we feel that our approach that limits
reduction in number of days of connection to no more than 15 percent will be protective
of this resource and those organisms dependent on them; however, as part of our
Appendix D vii