Discharge from Caloosahatchee River that enters Estero Bay
Michael J. Byrne
U.S. Geological Survey, Fort Myers, FL, USA
Estero Bay is a State aquatic preserve located about 3 miles south of the Caloosahatchee River along the coast of southwestern Florida. Five major tributaries flow into this long and shallow body of water including Hendry Creek, Mullock Creek, Estero River, Spring Creek, and Imperial River (fig. 1). Most water exchange between Estero Bay and the Gulf of Mexico is through four inlets through a chain of barrier islands; Matanzas Pass, Big Carlos Pass, New Pass, and Big Hickory Pass. Lands surrounding Estero Bay, including the barrier islands, are highly developed.
Figure 1. The Estero Bay watershed.
The hydrologic relation between the Caloosahatchee River and Estero Bay is poorly understood. The quantity and timing of freshwater flow into the Caloosahatchee River estuary has been altered due to anthropogenic activities. Principal discharge of water from the Caloosahatchee River into Estero Bay is through Matanzas Pass and Hurricane Pass. The discharge of the organic-rich water reduces water clarity and salinity in the northern part of Estero Bay.
Research reveals that the altered flow pattern impacts the Caloosahatchee River estuary and Estero Bay. The U.S. Geological Survey has undertaken a study to examine hydrology and salinity patterns in Estero Bay. Continuous salinity and flow measurements are being taken at Mullock Creek, Estero River, Imperial River, Matanzas Pass, Big Carlos Pass, and Big Hickory Pass (fig. 1). Surface-water salinity and temperature data also are being collected near the mouth of the Caloosahatchee River; at points in Matlacha Pass, Pine Island Sound, and Sanibel Island; and as far south as Wiggins Pass. These data, collected on a monthly basis, will be used to generate salinity maps. The salinity, flow and temperature data and additional discharge measurements may be collectively used to quantify the volume of water from the Caloosahatchee River that enters Estero Bay.
The Caloosahatchee River receives water from Lake Okeechobee (to the northeast) and from numerous tributaries. The tributaries north and south of the river drain a large basin consisting mainly of agricultural fields. The South Florida Water management District (SFWMD) regulates releases from Lake Okeechobee to the Caloosahatchee River. Flow on the Caloosahatchee River is controlled by a series of gated structures, operated by the U.S. Army Corps of Engineers (USACE). South Florida Water Management District permits discharge into the river in regulated pulses. The low-level pulse release is designed to limit negative impacts to the estuary. However, the river must also be maintained for navigation, water supply, and flood control. The USACE controls the locks on the river to meet these water demands. During summer 2002, the flow-way gates of the westernmost lock (Franklin Lock, S-79) were open almost continuously. Consequently, the Caloosahatchee River release occurred all summer, with highest flows during the period of discharge from Lake Okeechobee.
Water from the Caloosahatchee River was detected in the near-shore waters of the Gulf of Mexico as far south as Wiggins Pass, which is about 20 miles south of the mouth of the river. Salinity in the Gulf of Mexico substantially decreased during periods of large water releases from the river and was significantly lower off of the northern barrier islands than farther south where Caloosahatchee discharges had less effect. Freshwater entering the Gulf of Mexico from the Caloosahatchee tends to color the oceanic water brown or black. Further research will be required to understand the full extent of the freshwater discharge from the Caloosahatchee River.
Michael J. Byrne, U.S. Geological Survey, 3745 Broadway, Suite 301, Fort Myers, FL, 33901, Phone: 239-275-8448, Fax: 239-275-6820, , Hydrology and Hydrological Modeling