regions having pronounced daily tide-influenced variations in salinity (e.g., near the 
mouth of the Shark River estuary) or in offshore stations of Florida Bay where wind 
causes shifting of the water masses from basin to basin. These water masses at times 
have markedly different salinity characteristics. Areas having strong daily tide effect 
produce characteristic inverted L distributions when salinity is plotted against ground 
water levels. A different, but equally distinctive distribution occurs in non-tidal areas 
at the peak of the dry season. In both cases prediction tables based on regressions of 
ground water elevation against salinity must treat the rising (wet season) and falling 
(dry season) separately. Local rainfall apparently has little effect in changing salinity 
except at the end of dry seasons and drought, especially the latter. At such times the 
ground water level may be several tenths of a foot below mean sea level. If heavy rains 
fall on the estuary at such times measurable dilution may occur, especially in shallow, 
non-tidal areas, before downward seepage of the new surface water has raised the 
ground water levels sufficiently to cause discharge into the coastal estuaries. It was 
shown that an elevation of 0.6 ft above mean sea level is necessary at wells in the 
Shark River Valley before seaward movement of the freshwater line in the estuary 
could be detected. These studies indicate that ground water discharge is by far the most 
significant factor in moderating salinities of coastal waters in Everglades National 
Park. Furthermore, it is concluded that the average (i.e., 45 - 55 in yr 1 ) rainfall of 
southern Florida is not enough to cause the prolonged dilution of the estuaries as 
observed, and is insufficient to prevent an annual deficit between rainfall and evapo- 
transpiration. South Florida generally is classified as having a “tropical savannah 
climate’ where there is a relatively long and severe dry season, and the rainfall during 
the wet season is not sufficient to make up the water lost to evapo-transpiration during 
drought and dry seasons. Such areas experience a constant water shortage during the 
dry season, and hence must depend on water from outside the region, in this case, from 
the Kissimmee River-Everglades drainage, to compensate for water losses through 
evapotranspiration and to prolong the period of estuarine dilution. As a result of these 
factors, prolonged and significant dilution of the extensive estuarine, lagoon and marsh 
habitats, as well as of Florida Bay, is made possible only by prolonged and massive 
displacement southward of freshwater ‘down gradient" in the aquifer. Much of this 
water has its origin in the Conservation areas of the Central and South Florida Flood 
Control District north of the Park, or in the undeveloped area west of Miami and north 
of Homestead known as the “Southwest Dade“ area. This study spanned a period of 
severe drought and of heavier-than-average rainfall. Results of the study of Florida 
Bay salinity during this interval suggest that salinity may be expected to rise above 
50.0 °/oo whenever ground water elevations in the Homestead well fall below 200 ft 
above mean sea level. Furthermore salinity at the stations more than a mile offshore 
will fluctuate between 25.0 and 35.0 %o when ground water levels fluctuate between 
2.0 and 5.5 ft above mean sea level. However, if ground water levels increase to 6.0 ft 
above mean sea level an abrupt decline of 10.0 to 15.0 %o will occur in Florida Bay 
salinity. This suggests the height of ground water required to pour over the “sill“ 
formed by the Miami Oolite ridge between the Florida City-Homestead area and 
Mahogany Hammock in Everglades National Park. Furthermore, this suggests that the 
ridge is more permeable to lateral flow southward, near the present ground surface 
than deeper down under the ridge. The prediction method could be refined by use of 
more refined salinity measuring and recording systems. Such measurements should 
record salinity hourly and should be related to a current direction and velocity meter. 
With this kind of instrumentation at strategic locations in the respective estuaries and 
lagoons it would be possible to eliminate most of the errors observed. 
1 39 
