<P> From its origins to approximately the area of Sanford, the St. Johns flows north . It takes a sharp turn west near Sanford for a few miles--which is referred to as the St. Johns River offset, but shortly changes direction to flow north again . Geologists hypothesize that the west - flowing offset may have formed earlier than the north flowing portions, possibly during the late Tertiary or early Pleistocene era 66 to 12 million years ago . Some fracturing and faulting may also be responsible for the offset . Although seismic activity in Florida is mostly insignificant, several minor earthquakes have occurred near the St. Johns River, caused by the trough created by Pangaean rifting . </P> <P> All of Florida's abundant fresh water is the result of precipitation that returns to the atmosphere in a process called evapotranspiration that involves evaporation and transpiration of moisture from plants . As rains fall, most of the water is directed to lakes, streams, and rivers . However, a significant amount of fresh water is held underground but close to the surface in aquifers . A surficial aquifer consisting mostly of clay, shells, and sand is over a confining layer of denser materials . Wells are drilled in the surficial aquifer, which supplies better quality water in areas where the deepest aquifer has a high mineral content . Occasionally the confining layer is fractured to allow breaches of water to percolate down to recharge the layer below . The Floridan Aquifer, underneath the confining layer, underlies the entire state and portions of Georgia, Alabama, and South Carolina . It is particularly accessible in the northern part of Florida, serving as the fresh water source of metropolitan areas from St. Petersburg north to Jacksonville and Tallahassee . </P> <P> Acidic rainwater erodes the limestone and can form underground caverns . When the overlay of these caverns is particularly thin--less than 100 feet (30 m)--sinkholes can form . Where the limestone or sand / clay overlay dissolves over the aquifer and the pressure of the water pushes out, springs form . The upper and middle basins of the St. Johns River are located in a portion of the peninsula where the aquifer system is thinly confined, meaning springs and sinkholes are abundant . Springs are measured in magnitude of how much water is discharged, which is dependent upon season and rainfall . The greatest discharge is from first magnitude springs that emit at least 100 cubic feet (2.8 m) of water per second . There are four first magnitude springs that feed the St. Johns River: Silver Springs in Marion County, emitting between 250 and 1,290 cubic feet (7.1 and 36.5 m) / second; Silver Glen Spring straddling Marion and Lake Counties, emitting between 38 and 245 cubic feet (1.1 and 6.9 m) / second; Alexander Springs in Lake County, emitting between 56 and 202 cubic feet (1.6 and 5.7 m) / second; and Blue Spring in Volusia County, emitting between 87 and 218 cubic feet (2.5 and 6.2 m) / second . </P> <P> The St. Johns River lies within a humid subtropical zone . In summer months, the temperature ranges from 74 and 92 ° F (23 and 33 ° C), and between 50 and 72 ° F (10 and 22 ° C) in the winter, although drops may occur in winter months to below freezing approximately a dozen times . Water temperatures in the river correlate to the air temperatures . The average range of water temperatures is between 50 and 95 ° F (10 and 35 ° C), rising in the summer months . Where the river widens between Palatka and Jacksonville, wind becomes a significant factor in navigation, and both whitecap waves and calm surface waters are common . </P>

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