River sediment can serve as a sink or a secondary source of pollutants for overlying water. In this study, river sediments were collected in spring, summer, fall, and winter from seven locations of Ten Miles Creek (TMC), a main tributary river discharging into St. Lucie River, south Florida, USA, and analyzed for the concentrations and availability of phosphorus (P). River sediments had high water extractable P (W-P) 0.86-5.01 mg kg-1; Olsen P (O-P) 2.05-21.6 mg kg-1; Mehlich 3 P (M3-P) 3.71-58.4 mg kg-1; and Mehlich 1 P (M1-P) 4.63-291 mg kg-1. The highest concentrations of extractable P (W-P, O-P, M1-P, and M3-P) were observed in winter at the downstream and upstream locations. The values of DPS-M1 and DPS-M3 (the degree of P saturation (DPS) obtained from the determination of M1-extractable and M3-extractable Fe, Al, and P) of the river sediments were in the range of 12-3259, 9-2309, respectively. River sediments at the downstream locations had high DPS-M1 and DPS-M3, which were positively correlated with W-P (P<0.01) and O-P (P<0.01). Dissolved total P in water had positive correlations with M1 extractable P (P<0.01), water extractable P, and DPS-M1 (P<0.05) in the river sediments. These results suggested that river sediments in the TMC, especially at its downstream could serve as a potential secondary P source for overlying river water.