1.2. St. Johns River Basin Landscape

The LSJRB in northeast Florida has long been recognized as a treasured watershed – providing enormous ecological, recreational, socioeconomic, and aesthetic benefits. However, during recent years, it has also been recognized as a threatened watershed, which is critically in need of resource conservation, water quality improvement, and careful management.

1.2.1 Geopolitical Boundaries

For management purposes, the entire St. Johns River watershed is commonly divided into five basins: the Upper Basin (southern, marshy headwaters in east central Florida), the Middle Basin (the area in central Florida where the river widens, forming Lakes Harney, Jesup, and Monroe), the Lake George Basin (the area between the confluence of the Wekiva River and St. Johns River and that of the Ocklawaha River and the St. Johns River), the Lower Basin (the area in northeast Florida), and the Ocklawaha River Basin (the primary tributary for the St. Johns River). The LSJRB is the focus of this State of the River Report.

As a constant, this Report defines the LSJRB in accordance with the SJRWMD definition: “the drainage area for the portion of the St. Johns River extending from the confluence of the St. Johns and Ocklawaha rivers near Welaka to the mouth of the St. Johns River at Mayport” (SJRWMD 2008; Figure 1.1).

The LSJRB includes portions of nine counties: Clay, Duval, Flagler, Putnam, St. Johns, Volusia, Alachua, Baker, and Bradford (Brody 1994). Notable municipalities within the Lower Basin include Jacksonville, Orange Park, Green Cove Springs, and Palatka (Figure 1.1).

The LSJRB covers a 1.8 million-acre drainage area, extends 101 miles in length, and has a surface area of water approximately equal to 115 square miles (Adamus, et al. 1997; Magley and Joyner 2008).

1.2.2. Existing Land Uses

The LSJRB, including all aquatic and adjoining terrestrial habitats, consists of approximately 68% uplands and 32% wetlands and deepwater habitats (Figure 1.2, see Appendix 1.2.2.A. for acres and definitions of categories).

Figure 1.2
Figure 1.2 Total percentages for land, wetland, and deepwater habitats within the Lower St. Johns River Basin, Florida. (Source: SJRWMD Wetlands and Deep Water Habitats GIS Maps, 1972-1980; SJRWMD 2007a)

Within the LSJRB in 2004, the dominant land covers were upland forests (35%) and wetlands (24%), and 18% was considered urban and built-up. Since the 1970s, the proportion of the total basin designated as upland forests and agriculture has decreased, while the proportion designated as urban and built-up has increased (see Appendix 1.2.2.B.; SJRWMD 2007).

1.2.3. Ecological Zones

The LSJRB is commonly divided into three ecological zones based on expected salinity differences (Hendrickson and Konwinski 1998; Malecki, et al. 2004). The mesohaline riverine zone is the most northern ecological zone in the LSJRB, stretching from the Atlantic Ocean to the Fuller Warren Bridge. The mesohaline riverine zone is typically deeper and well-mixed with an average salinity of 14.5 parts per thousand (ppt) and a fast flow rate. South of the Fuller Warren Bridge, the St. Johns River widens into a broad, shallow, slow-moving, tidal area called the oligohaline lacustrine zone. This zone extends from the Fuller Warren Bridge to Doctors Lake and has an average salinity of 2.9 ppt. South of Doctors Lake to the confluence of the St. Johns and Ocklawaha rivers near Welaka, the LSJRB transitions into the freshwater lacustrine zone. This zone stretches through the Middle and Upper Basins of the St. Johns River as well. The freshwater lacustrine zone is lake-like, has an average salinity of 0.5 ppt, and experiences tidal fluctuations that are lower than those observed in the other ecological zones.

1.2.4. Unique Physical Features

The St. Johns River is unique and distinctive due to a number of exceptional physical features.

The St. Johns River is the longest river in Florida. Stretching 310 miles and draining approximately 9,430 square miles, this extensive river basin drains about 16% of the total surface area of Florida (DeMort 1990; Morris IV 1995).

The St. Johns River flows northward. The result of this northward flow is that the Upper St. Johns actually lies south of the Lower St. Johns (DeMort 1990). The St. Johns River is one of the few rivers in North America to flow north.

The St. Johns River is one of the flattest major rivers in North America. The headwaters of the St. Johns River are less than 30 feet above sea level. The river flows downward on a slope ranging from as low as 0.002% (Benke and Cushing 2005) to about 1% (DeMort 1990). This slope is governed by the exceptionally flat terrain of the drainage basin and most of the decline occurs in the first 100 miles of the river. In fact, the river bottom at the mouth of Lake Harney is below sea level (Bowman 2009). This extremely low gradient contributes to a typically slow flow of the St. Johns River. This holds back drainage, slows flushing of pollutants, and intensifies flooding and pooling of water along the river creating numerous lakes and extensive wetlands throughout the drainage basin (Durako, et al. 1988). The retention time of the water, and its dissolved and suspended components, in the river is on the order of three to four months (Benke and Cushing 2005). High retention times of pollutants have severe impacts on water quality.

The Lower St. Johns River is a broad, shallow system. The average width of the Lower St. Johns River from Lake George to Mayport is one mile, although the flood plain reaches a maximum width of ten miles (Miller 1998). The average depth of the river is 11 feet (Dame, et al. 2000). The variability in width of the river can result in different water flow patterns and conditions on opposing banks of the river (Welsh 2008).

The St. Johns River receives saltwater from springs. Several naturally salty springs feed into the St. Johns River Drainage Basin. The most significant inputs of salty spring water originate from Blue Springs, Salt Springs, Silver Glen Springs, and Croaker Hole Spring (Campbell 2009). Inputs from these salty springs cause localized areas of elevated salinity (>5 ppt) in otherwise freshwater sections of the river (Benke and Cushing 2005). The amount of flow from springs is highly variable and dramatically affected by droughts (Campbell 2009).

The St. Johns River drains into the Atlantic Ocean. The average discharge of water at the mouth of the St. Johns River is 8,300 cubic feet per second (Miller 1998) or 5.4 billion gallons per day (Steinbrecher 2008). However, this flow rate is dwarfed by the volume of tidal flow at the mouth of the river, which is estimated to be approximately seven times greater than the freshwater discharge volume (Anderson and Goolsby 1973). This difference often causes “reverse flow,” or a southward flow, up the river. Reverse flow has been detected as far south as Lake Monroe, 160 miles upstream, and is influenced as much by weather conditions as by ocean tides (Durako, et al. 1988). Natural water sources for the St. Johns River are direct rainfall, rainfall from runoff, underground aquifers, and springs. Continual input from springs and aquifers supplies the river with water that discharges into the Atlantic Ocean, despite drought periods or seasonal declines in rainfall (Benke and Cushing 2005). Water quality depends on the primary sources of water at any given time.

The Lower St. Johns River is a tidal system with an extended estuary. The tidal range at the mouth of the river at Mayport, Florida is about six feet (McCully 2006). The Atlantic Ocean’s tide heights are large compared to the slope of the St. Johns River, and at times, can produce strong tidal currents and mixing in the northernmost portion of the river. The St. Johns River is typically influenced by tides as far south as Lake George, 106 miles upstream (Durako, et al. 1988). During times of drought when little rainwater enters the system or extreme high tides, river flow-reversal can occur as far south as Lake Monroe, 160 miles upstream (Durako, et al. 1988). Tidal reverse flows occur daily in the LSJR, and net reverse flows, as much influenced by winds as by tides, can occur for weeks at a time (Morris IV 1995).

The salinity of the St. Johns River is heavily affected by seasonal rainfall patterns and episodic storm and drought events. In general, there is a predictable seasonal pattern of freshwater input from rainfall into the Lower St. Johns River, with the majority of rain falling during the wet season from June to October (Rao, et al. 1989). However, this seasonal pattern of rainfall can be overridden by less predictable, episodic storm events, i.e., hurricanes, tropical storms, or nor’easters, or drought events, like the droughts of the early 1970s, the early 1980s, 1989-1990, and 1999-2001 (DEP 2010d). In turn, surges of freshwater from heavy rainfall tend to reduce salinity levels in the river. Increased salinity occurs during periods of drought, when there is a deficit of fresh rainwater into the river. Thus, rainfall can prompt a chain of events in the river, where changes in salinity lead to impacts on aquatic plants and animals. Simplified examples of several sequenced events are illustrated below (Figure 1.3).

Figure 1.3
Figure 1.3 Simplified example of sequenced events that can occur in the Lower St. Johns River Basin stimulated by changes in rainfall.

The St. Johns River can be influenced by local wind direction. Surface stress of local winds upon the river plays a secondary role compared with remote winds on the ocean that affect the river’s flow. However, these local winds can cause flow enhancements. South winds blowing to the north accelerate the flow of water toward the ocean, if the flow is not opposed by a strong tidal current. Similarly, north winds can push river water back upstream (Welsh 2008). Strong sustained north winds from fall nor’easters or summer hurricanes can push saltwater up the river into areas that are usually fresh. Although considered a natural occurrence, reverse flow of the river can impact flora and fauna with low salinity tolerances and cause inland areas to flood.

The St. Johns River is a dark, blackwater river. Southern blackwater rivers are naturally colored by dissolved organic matter derived from their connections to swamps, where plant materials slowly decay and release these organic materials into the water (Brody 1994). The Dissolved Organic Matter (DOM) limits light penetration, and therefore photosynthesis, to a very shallow layer near the surface of the river.