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

third highest flow occurred early in 1960 (March 17, 1960) presumably in
connection with an El Nino event. While the fourth highest flow on record
occurred on July 30, 1960, it is likewise interesting to note that the fifth highest
daily flow (of 18,508 observations) occurred on March 20, 1959. Although flows
below 10 cfs were recorded during the 2000 drought, the lowest recorded flow
was on May 17, 1952 at 3.9 cfs. The period of record mean daily flow is 153 cfs
and the median daily flow is 90 cfs.
The South Prong gage near Lithia (USGS # 2301300) was established on
January 1, 1963, and is located 7.6 miles upstream of the confluence with the
North Prong. Although spanning 40 years, the flow record is considerably shorter
at this site than at either the Lithia or North Prong site. The single highest flow
recorded at this site was 2,430 cfs on August 14, 1967; the second highest flow
at 2,110 cfs was recorded on June 22, 2003. Flows of zero cfs were recorded in
May and June of 2000 and 2001, the period of record drought. During the period
of record (ending Sep. 20, 2003) the mean and median daily flows were 101 and
57 cfs, respectively.
2.3.4.2 Alafia River Flows
Annual percent exceedance flows were determined for each year in the period of
record at gage sites located on the mainstem near Lithia, on the South Prong of
the Alafia near Lithia, and on the North Prong of the Alafia at Keysville. The
South Prong gage measures discharge from a 107 square mile watershed, and
the North Prong gage measures discharge from a 135 square mile watershed.
The Lithia gage measures discharge from an area of 335 square miles. Because
the Lithia gage is located downstream of the confluence of the North and South
Prongs, its watershed encompasses the combined area of the gaged North and
South Prongs (i.e., 242 square miles) plus an additional 93 square miles. Based
on relative watershed size, the gaged area of the South Prong should contribute
approximately 32% of the flow as measured at the Lithia gage, and the gaged
area of the North Prong should contribute 40% of the flow as measured at the
Lithia gage.
Selected percent exceedance flows for the period of record at the Alafia, North
Prong and South Prong gages are shown in Figures 2-17 to 2-19. Apparent at
the Alafia gage at Lithia is a substantial increase in low to median flows (99% to
50% exceedance flows) beginning around 1960. Both Stoker et al. (1995) and
Hickey (1998) noted this increase in flow; Hickey (1998) postulated that the
increases could be attributed to phosphate mining related discharges. Inspection
of USGS water quality data collected at the Lithia gage on an approximate
quarterly basis validates this supposition (see Figure 2-20). Exceptionally high
phosphorus concentrations and elevated fluoride concentrations are obviously
mine related, since fluoride is found in association with apatite, the phosphate
ore that is mined. What is evident from water quality data is a substantial
2-40