High levels of fecal coliforms, total phosphates, biochemical oxygen demand, and ammonia and nitrate nitrogens impacted water quality primarily during fall, winter, and spring. These constituents are occasionally present during summer low flow conditions. High water temperatures and eutrophication is evident at this site during summer months. As a result, high pH values, high dissolved oxygen levels, and high biochemical oxygen demand impair water quality. In comparing OWQI values from the most upstream to most downstream of Coast Fork Willamette River sites, it is interesting to note that water quality remains relatively steady throughout the river in the fall, winter, and spring.
Water quality improves from upstream to downstream during the summer months.
This indicates diminishing sources of pollution towards the mouth of the river. The Middle Fork Willamette Subbasin has more flow controlling reservoirs than any other subbasin in the Willamette Basin. Though presenting a barrier to fish passage, the system of dams and reservoirs maintains relatively high flows and low temperatures throughout the year, compared to the Coast Fork Willamette River. The Middle Fork Willamette River at Jasper is occasionally impacted by high levels of total phosphates and biochemical oxygen demand during heavy precipitation. This indicates pollution from run-off, which is present in any river system.