54 REPORT OF INVESTIGATIONS No. 50
The average slope of the St. Johns River is less than 0.3 of a foot per mile in its approximately 26-mile reach along the border of Orange County. At flood stages, the river falls from an altitude of about 17.5 feet at Lake Poinsett to about 10.5 feet at the northern county line. At the minimum stages in 1945, the river fell from 8.0 feet to minus 0.4 foot in this reach. Figure 17 shows probable flood altitudes for the St. Johns River for selected recurrence intervals.
Stage and discharge records have been collected at St. Johns River near Christmas (station 35) since December 1933 and at St. Johns River near Cocoa (station 36) since October 1953. The average discharge for the period of record at station 35 was 1,379 cfs. For the 10-year period October 1953 to September 1963, the average discharge at station 35 was 1,463 cfs; and at station 36, 1.237 cfs. The maximum flow during the period of record at station 35 was 11,700 cfs in October 1953. There was no flow at station 35 for periods during March, April, and June 1939. Average yearly runoff is 13.2 inches at station 35 and 12.8 inches at station 36. The slightly higher yield at station 35 may be due partly to the absence of lakes, where evaporation losses are high, and partly to upward seepage of artesian water in the area between the two stations. Curves 1 and 2 (fig. 14) are flow-duration curves for stations 35 and 36. As indicated by the curve, about 99 percent of the time flow at station 35 exceeds that at station 36 but 1 percent of the time evaporation and transpiration demands on the river exceed the seepage into the river causing a loss in flow between the stations. Figure 22 shows the magnitude and frequency of annual minimum flow for selected durations at station 35.
Analyses of water collected daily from the St. Johns River near Cocoa (station 36) from October 1953 to September 1960, and a continuous record of its conductivity since June 1959 show that the quality of the water varies greatly. Table 8 gives ranges for the various dissolved constituents.
Except for color the quality of the water in the St. Johns River near Cocoa is good during normal and high flows. During droughts when low flows occur, the water in the St. Johns River becomes highly mineralized. During extended droughts, as occurred in 1962, very little water flows into the St. Johns River above the Wekiva River. Most of the low flow comes from seepage of highly mineralized artesian water from the Floridan aquifer. Along the reach of the river adjacent to Brevard County, the Floridan aquifer