It is possible that the Myrtle Point STP could influence water quality at the North Fork Coquille River monitoring site under the right tidal conditions. Water quality impacts at this site include high levels of fecal coliforms, total solids, nitrate nitrogen, total phosphates and biochemical demand during high flows. Occasionally during low flow, very high levels of biochemical oxygen demand contribute to low levels of dissolved oxygen. Water quality impacts are even more complicated at the next monitoring site, the mainstem Coquille River at Strudivant Park Dock. This site is situated downstream of agricultural non-point sources of concern, log ponds, and the Coquille STP outfall.
Tides also influence water quality here.
High levels of fecal coliform, total solids, and total phosphates are accompanied with moderately high levels of ammonia and nitrate nitrogens and biochemical oxygen demand throughout the year. During periods of low flow, high temperatures combined with high levels of oxygen demand lead to the depletion of available dissolved oxygen. The Sixes subbasin includes the Sixes and Elk Rivers. Water quality conditions at the Sixes River at US Highway 101 are generally very good or excellent. However, during periods of heavy precipitation and high flows, high concentrations of total phosphates, total solids, and biochemical oxygen demand, indicating erosion and field runoff, inhibit water quality.