Orthophosphate concentrations in the Tualatin River of northwest Oregon have historically been high enough for the formation of seasonal algal blooms in the lower slow moving stretches of the river. Past work to decrease phosphate levels concentrated on limiting agricultural runoff and reducing effluent from water treatment plants, yet phosphate levels have remained high. Close examination of the Willamette Silt and underlying Hillsboro Formation in the Tualatin Valley has revealed that phosphate is leaching from the substrata into the overlying drainage system through ground-water discharge. Hillsboro Formation samples from subsurface borings as deep as 330 m contain up to 3.17 mg/1 orthophosphate as measured by saturated pastes. Three distinct zones of phosphate concentrations are recognized in the HBD-1 core drilled at the Hillsboro airport; the top 65 m average 0.3 mg/1 orthophosphate, the next 60 m average 1.22 mg/1, and the bottom 138 m average 0.1 mg/1. Reductions in orthophosphate concentrations below a depth of 150 m correspond with the presence of small vivianite nodules and crystals, and increased abundances of magnetite, both which persist to the base of the Hillsboro Formation. Changing redox conditions with depth along with phosphate complex adsorption onto iron oxides in the shallow zone best explain the observed relationships between phosphate, vivianite, and magnetite concentrations in the sediments. Observations in other borings from the central and western Tualatin Basin support the above hypothesis. Naturally large phosphate concentrations leaching from the Hillsboro Formation and into the Tualatin River drainage system will always keep the river at risk of accelerated seasonal algal growth.