These wastes enter the river as a result of runoff and erosion associated with precipitation and high river levels. Occasional STP overflows may occur during periods of heavy precipitation. These conditions persist in the summer, when high water temperatures are added to the list of concern. During the last ten years, Philomath STP facilities were improved and the severity and frequency of water quality impacts to Marys River have lessened. The monitoring site on the Willamette River at Oregon Highway 34 in Corvallis is approximately one mile downstream of the confluence with Marys River.
Water quality is impacted more frequently in the fall, winter, and spring than in the summer.
Wet weather impacts include high levels of fecal coliforms, total phosphates, nitrate and ammonia nitrogens, total solids, and biochemical oxygen demand. Summer impacts include high temperature, nitrate nitrogen, and total solids. During the last ten years, the severity and frequency of water quality impacts to the Willamette River at Corvallis have lessened. STP effluents have improved in quality as a result of the addition of secondary treatment to STP's in the region. Industrial point sources have been removed or improved.