cause abnormally hiqh water without the 

 usual ebb. 



The air circulation over the Gulf of 

 Mexico is primarily anticvclonic (clock- 

 wise around an atmospheric high-pressure 

 region) during much of the year. However, 

 strong air masses of continental origin 

 often move through the northern Florida 

 area, especially during the winter. From 

 November to March, an average of 30 to ^0 

 polar air masses penetrate the Gulf each 

 year. Storms are usually formed along 

 slow-moving cold fronts in winter. 

 Tropical storms or hurricanes may occur in 

 summer and early fall. Lesser storms 

 often occur as extratropical cyclones, 

 which tend to move across the Gulf from 

 west to northeast during winter oeriods 

 (Jordan 1973). Winter storms tend to be 

 more pervasive in a geographic sense, 

 while summer storms are often intensive, 

 short-lived, localized events. The 

 likelihood of the occurrence of a 

 hurricane in the northeast Gulf is about 

 once every 17 years with fringe effects 

 about once every S years (Clewell 1^78). 

 The last hurricane to hit Apalachicola, 

 Hurricane Agnes, occurred in June 197?. 

 Overland (1975) showed that basin 

 orientation (relative to wind direction, 

 headlands, and marsh areas) can produce 

 variations in surge heights, which are 

 responsible for much damage. Livingston 

 (unpublished data) found that Hurricane 

 Agnes had no sustained effect on water 

 quality or the biota o^ the Apalachicola 

 estuary. 



''.3. HYDROLOGY 



?.3.1. Freshwater Input 



The Apalachicola River has the 

 highest flow rate (690 m^ sec"l at 

 Chattahoochee, Florida; 1958-1^80) and 

 broadest flood olain (450 km? of bottom- 

 land hardwood and tupelo-cypress forests) 

 of any river in Florida (H. M. Leitman et 

 al. 1982). Apalachicola River discharge 

 accounts for 35^ of the total <'reshwater 

 runoff on the west coast of Florida 

 (McNulty et al. 1'572). Seasonal variation 

 (Figure Q) is high, with peak flows from 

 January through April and low flows from 

 September through November. The absence 

 of a summer river-flow peak (despite rain- 

 fall peaks in the basin at this time) may 



be related to higher evapotranspiration 

 rates in the vegetation of the watershed 

 (Livingston and Loucks l'^78). A spectral 

 analysis using data from 1920 to 1977 

 (Figure 10) indicated river-flow cycles on 

 the order of 6-7 years (Meeter et al. 

 1<^79). Indications of longer-term cycles 

 were shown along with the abnormally low 

 river flow during the mid-1950's. 



In a cross-spectral analysis of 

 Georgia rainfall with river flow, the two 

 patterns were in phase (Meeter et al . 

 1979; Figure °) . The analysis indicated 

 that the Apalachicola River flow patterns 

 more closelv resembled cycles of Georgia 

 rainfall than they did those of Florida 

 rainfall. This pattern should be expectd 

 since only 11.6% of the drainage basin is 

 in Florida, and the remainder is in 

 Georgia. Stage fluctuations vary greatly 

 from upper to lower river with the 

 narrowest ranges (from peak to low) at 

 downstream stations (H. M. Leitman et al. 

 1982). Such flooding patterns are 

 essential to elements of the hydrology of 

 the estuary. 



Floodplain inundation varies with 

 location on the river and reflects the 

 influence of natural riverbank levees 

 (H. M. Leitman et al. 1982). Natural 

 levees within the flood plain are 

 inundated only at high stages of river 

 flow. The level of the water table also 

 depends on river stage. Fluctuations are 

 damped by water movement through flood- 

 plain soils. The levees of the upper 

 river, where there is a greater range of 

 water fluctuation, are higher than those 

 in the lower river where the flood plain 

 is guite flat. Flood depths tend to 

 decrease from the upper to the lower river 

 and rates of flow in the upper river 

 floodplain are generally less than those 

 along the middle and lower reaches of the 

 river. The height of the natural levees 

 and the size and distribution of breaks in 

 the levees all control the hydrological 

 conditions of the river flood plain. Such 

 hydrological conditions, in turn, control 

 the form and distribution of floodplain 

 vegetation (H. M. Leitman et al . 1982). 



2,3.2. Tides and Currents 



Franklin County straddles a region of 

 transition between the diurnal tides of 



13 



