during tidal cycles. At Crystal River during normal tidal cycles the maximum positive (downstream) and negative (upstream) flow is about 4,000 cfs. During 0 Hurricane Dora in September, 1964, the maximum instantaneous negative flow was estimated to be more than 10,000 cfs--largely caused by wind-driven tides.
WHERE DOES THE WATER COME FROM?
The vast flow of the springs is derived from rain-about 55 inches annually-that falls on about 2,300 square miles east of the springs, figure 10. About 38 inches of the rain returns to the atmosphere annually by the evapotranspiration-evaporation from water, soil, and plant surfaces and by transpiration by vegetation.
Very little water (as streamflow) leaves the area immediately west of the western topographically defined drainage divide of the Withlacoochee River (fig. 10). This 570 square mile area is almost devoid of surface drainage. Surface runoff (runoff per square mile of topographic drainage area) from the Withlacoochee River is low in comparison to adjacent stream basins. Much of the rain that falls on the area enters the Floridan (limestone) aquifer either by percolation through the soil zone into the limestone or by drainage through sinkholes, moves generally westward toward the large coastal springs, and reappears as springflow and seepage into the coastal rivers.
The Floridan aquifer, which is composed of more than 1,000 feet of limestone and dolomite in the "Sun Coast" area, is one of the most productive aquifers in the United States and probably the world. The aquifer transmits water beneath the area to the springs near the coast and is the source of virtually all water used locally. Figure 11 shows the piezometric surface of the Floridan aquifer in the coastal springs area. The contour lines represent the height above mean sea level to which water would rise in tightly cased wells that penetrate this aquifer, and the arrows indicate the general direction of flow through the aquifer.
The top of the aquifer is about 60 to 80 feet below land surface in the central part of the state but is at or near land surface close to the coast. The gentle slope of the limestone aquifer, its high calcium carbonate content and natural porosity, the small amount of surface runoff, the dense vegetation and humid climate, and active circulation of ground water create an environment in this area of Florida- that is most favorable for solution of the limestone. The results of limestone solution is cavity formation and the development of