Sediment concentrations and loads in the Loxahatchee River estuary, Florida, 1980-82

A study was conducted to estimate the magnitude and composition of sediment loads and the general spatial and temporal patterns of sediment transport in the Loxahatchee River estuary.Mean concentrations of suspended sediment generally were higher in the Jupiter Inlet area than in the remainder of the embayment area, ranging from 14 to 60 milligrams per liter at maximum tidal flows.Organic material accounted for 13 to 45 percent of the suspended sediment on incoming and outgoing tides at sites in the inlet area.During slack tides, much of the sediment stayed in suspension, and concentrations at sites near Jupiter Inlet remained about 9 to 12 milligrams per liter.Mean concentrations in the estuary embayment area usually ranged from 8 to 20 milligrams per liter during outgoing tides and from 10 to 18 milligrams per liter during incoming tides.Organic material generally comprised 24 to 50 percent of the suspended sediment in the embayment area.Concentrations of suspended sediment in the tributaries varied with season and weather conditions.Mean suspended-sediment concentrations during the study period ranged from 8 to 23 milligrams per liter, but concentrations in selected tributaries immediately following Tropical Storm Dennis in August 1981 increased as much as 16 times over concentrations before the storm.Organic material accounted for an average of 28 to 46 percent of the suspended sediment during the study period.Suspended-sediment loads from the tributaries were highly seasonal and storm related.For example, during a 61-day period of above-average rainfall that included Tropical Storm Dennis, the 5 major tributaries (Loxahatchee River at State Road 706, Cypress Creek, Kitching Creek, Hobe Grove Ditch, and Canal 18 at control structure 46) discharged 926 tons (short; 2,000 pounds) of suspended sediment to the estuary, accounting for 74 percent of the input for the 1981 water year (October 1, 1980, through September 30, 1981) and 49 percent of the input for the 20-month study period.Loads of suspended sediment increased from 9 tons (short) during a 5-day prestorm period to 302 tons (short) during a 5-day, stormwater-runoff period.Runoff from the storm accounted for 24 percent of the suspended sediment transported by the tributaries during the 1981 water year.Suspended-sediment loads at Jupiter Inlet and at the mouth of the estuary embayment on both incoming and outgoing tides far exceeded tributary loads, but the direction of long-term, net-tidal transport was not determined.Tidal loads on outgoing tides ranged from 124 to 1,060 tons (short) at the inlet and from 97.5 to 175 tons (short) at the mouth of the embayment Tidal loads on incoming tides ranged from 113 to 724 tons (short) at the inlet and from 86.5 to 183 tons (short) at the mouth of the embayment.Total loads from the tributaries during an average 6-hour tidal cycle during the 1981 water year was about 0.83 ton (short).