Salinity characteristics and distribution and effects of alternative plans for freshwater withdrawal, Little Manatee River estuary and adjacent areas of Tampa Bay, Florida

Abstract

The Little Manatee River, a coastal stream that flows into Tampa Bay, may be used as a source of freshwater. Fifty percent reduction of streamflow will result in upstream movement of saltwater. Data on streamflow, tide stage, and specific conductance describe the salinity distribution of the Little Manatee River estuary. Vertical conductivity profiles indicate that the river is vertically homogeneous during low flows. The maximum upstream location of the saltwater-freshwater interface in the river was described by multiple regression analysis using mean daily streamflow and high-high tide data. The coefficient of determination is 0.94 with a root mean square error of +0.4 mile. The analysis for the river ranged from mean daily discharges of 42 to 118 cubic feet per second, high-high tides of 0.81 to 2.48 feet above sea level, and a location of the observed saltwater-freshwater interface of from 5.8 to 10.4 miles above the reference station at Shell Point. The location of the river where the estuarine system ends and the riverine system begins is about 9.9 miles above the reference station at Shell Point. The 800-micromho conductivity line (260 milligrams per liter chloride) demarcates the estuarine and riverine systems. A duration analysis of conductivity indicates that the saltwater-freshwater interface exceeded the 24th Street site, 9.7 miles above the reference station, about 17 percent of the days for the period of study A duration analysis of the computed location of the interface indicates that the location will exceed 9.7 miles about 12 percent of the days. Reduction of streamflow for the 90-day, 2-year recurrence-interval low flow (30.7 cubic feet per second) by 50 percent would cause the maximum intrusion of the interface to move upstream from mile 10.4 to mile 11.1 for a mean high-high tide of 1.65 feet above sea level. Fifty percent reduction of the 90-day, 20-year recurrence-interval low flow (9.37 cubic feet per second) would move the saltwater-freshwater interface 0.2 mile upstream, or from river mile 11.4 to mile 11.6 for a high-high tide of 1.65 feet.