Home Methods of Study Bathymetry Freshwater Flow Specific Conductance Summary & References

Freshwater Flow

Estuaries are complex systems at the interface of oceanic and riverine environments. The mixing of freshwater runoff primarily from Cape Coral and incoming saltwater tides from San Carlos Bay and Charlotte Harbor (figs. 2 and 3) allows many species of animals and plants to flourish in the estuary. Freshwater runoff from Cape Coral is mainly because of canals that are designed to collect runoff and distribute it as sheetflow through two spreader canal systems into Matlacha Pass or from canals that discharge directly into the Caloosahatchee River.

The canals within the Cape Coral drainage basin are divided into two different drainage systems: the north Cape Coral spreader canal system (fig. 2) and the south Cape Coral spreader canal system (fig. 3). State Road 78, extending from Cape Coral in the east to the city of Matlacha, divides these two canal systems.

The north Cape Coral canal system has a drainage area of about 46 mi² east of State Road 765 and consists of four freshwater canals that discharge into spreader canals (fig. 2). This canal system, referred to as the north Cape Coral spreader canal system, is subject to tidal overtopping during extreme high tides and may contain brackish to salty water (1,301 to 28,800 µS/cm of specific conductance) at and near the canal bottoms. Several breeches in the north Cape Coral spreader canal system allow freshwater to discharge as point sources into the estuary.

The south Cape Coral spreader canal system has a drainage area of about 20 mi² upstream of the salinity weirs and consists of five canals (fig. 3). Four of these canals (not shown) discharge into the Caloosahatchee River. The Aries Canal, with an indeterminate drainage basin because of the extensive interconnection of canals and weir placement, discharges freshwater into Matlacha Pass (fig. 3).

Most of the freshwater flow into Matlacha Pass comes from the spreader canal systems during the wet season. Other sources of inflow are from direct rainfall into Matlacha Pass and freshwater runoff from Pine Island. For the 1990 water year, more than 67 percent of the total measured flow from the north Cape Coral spreader canal system and about 80 percent of the total measured flow from the south Cape Coral spreader canal system occurred during the wet season. Freshwater flow into Matlacha Pass from the north Cape Coral spreader canal system averaged 113 ft³/s for the period October 1987 to September 1992 and 49.6 ft³/s for the 1990 water year. Freshwater inflow from the Aries Canal of the south Cape Coral spreader canal system averaged 14.1 ft³/s for the period October 1989 to September 1992 and 5.73 ft³/s for the 1990 water year. Hydrographs showing daily mean discharge from the Aries Canal and the north Cape Coral spreader canal system are depicted in figures 9 and 10, respectively.

Figure 9. (above) Daily mean discharge from the Aries Canal. [larger image]	Figure 10. (above) Daily mean discharge from the north Cape Coral spreader canal system (sum of daily mean discharge at Gator Slough, Horseshoe, Hermosa, and Shadroe). [larger image]