Middle St. Johns - Florida Department of Environmental Protection

Water Quality Assessment Report: Middle St. Johns
37
The St. Johns River throughout its length has a low gradient, averaging
about 0.1 foot per mile (Campbell et al., 1993). Because of the low
stream gradient, tidal reversal may occur as far upstream as Lake Monroe
(DeMort, 1991), although this lake is well over 100 miles from the Atlantic
Ocean. It is more typical for daily tidal reversals to occur as far up from
the river mouth as Lake George (DeMort, 1991).
The river receives ground water flow from several springs or spring-fed
rivers. The most notable spring-fed river is the Wekiva River, which starts
as a discharge from Wekiwa Springs and enters the St. Johns just north
of Lake Monroe. Between Lake Monroe and Lake George, the St. Johns
River receives flow from Blue, DeLeon, and Alexander Springs. Alexander
Springs enters the St. Johns River at Lake Woodruff. Juniper, Silver
Glen, and Salt Springs discharge to creeks along the western shore of Lake
George. Croaker Hole Spring discharges through the riverbed into Little
Lake George. Lake Monroe also receives flow from its tributaries, Bethel
Creek and DeBary Drain.
Between Lake Harney and Lake Monroe, the river is somewhat saline
because of shallow marine salt deposits (DeMort, 1991). The discharge
from many of the springs also contains a fairly high concentration of salts,
producing a measurable salinity. The source of this salt is trapped relict
seawater or deeper brackish water released by upwelling through fractures
or faults in the aquifer (Toth, 1999). Salt is trapped in the aquifer most
probably from incomplete flushing of saline water that entered the Florida
aquifer during the last rise of sea level that covered Florida (Knochenmus
and Hughes, 1976). Springs with high salt concentrations are Croaker
Hole in the riverbed within Lake George; Salt, Silver Glen, Sweetwater,
and Alexander Springs near Lake George; Gemini and Blue Springs north
of Lake Monroe; and Ponce de Leon Spring near Lake Woodruff (Toth,
1999). Salinity in these areas is high enough to support blue crabs.
Tributary Watersheds
There are nine major tributary watersheds, including three drainage
basins for the large lakes that are part of and include the main stem of the
St. Johns River. These are the planning units for this basin. Tributary
watersheds that are not part of the river’s main stem are, from south to
north, the Econlockhatchee River, Lake Jesup, Deep Creek, Wekiva River,
Lake Kerr, and Alexander Springs. Descriptions of the tributary watersheds
were taken from the Middle St. Johns River Surface Water Improvement
and Management (SWIM) Plan (SJRWMD, 2002) unless otherwise noted.
The Alexander Springs watershed drains almost 100 square miles of
largely undeveloped land. Other than isolated lakes, Alexander Springs
Creek is the major stream in this watershed. It receives its discharge from
Alexander Springs and several small tributaries that, along with the spring,
form the headwaters. Alexander Springs Creek enters wetlands along the
St. Johns River at Lake Woodruff.
The Deep Creek watershed, which includes Lake Harney, is an aggregate
of several smaller tributary watersheds that drain to the St. Johns
River. The watershed drains 274 square miles.