Oregon Water Quality Index Report for Mid Coast Basin

Water Years 1986-1995

Water quality conditions in the Mid Coast basin do not vary as widely as those in the North Coast basin. Smaller intercoastal valleys and bays result in less room for population growth and less variety of land/water use. Comparing minimum seasonal Oregon Water Quality Index (OWQI) values (Table 1), water quality ranges from fair (Siuslaw River site) to excellent (Siletz River site). During the period of Water Years 1986-1995, water quality data were collected routinely by the laboratory beginning in 1992.

Siletz-Yaquina Subbasin

The Siletz-Yaquina subbasin includes the Salmon, Siletz, and Yaquina Rivers. Nitrate nitrogen is the primary limiting factor on water quality throughout the Mid Coast basin. High levels of nitrates accompanied by increases in total phosphates, total solids, and biochemical oxygen demand, appear during periods of heavy precipitation. Nutrient-rich erosion products deposited during storm events place a high demand on available dissolved oxygen in the water. These products may be naturally occurring, but are more likely the result of non-point source pollution. In the Siletz-Yaquina subbasin, the concentration of nitrogen, range of variation in water quality, and impact on OWQI scores increase as follows: Siletz River, Salmon River, Yaquina River (Table 1).

Alsea Subbasin

The Alsea River is monitored at Thissell Road (Mike Bauer Roadside Park). Water quality here is similar to that in the Siletz River. Winter precipitation sends nutrient-rich erosion products to the streams. On the average, water quality is good in the fall, winter, and spring, and improves to excellent in the summer (Table 1).

Siuslaw Subbasin

The Siuslaw River is monitored at Highway 126 in Mapleton. During fall, winter, and spring, water quality conditions in the Siuslaw are similar to those in the Alsea. When flow is very low in summer, a tidally produced saltwater wedge intrudes as far upstream as Mapleton. Like the Klaskanine River in the North Coast basin, this produces high concentrations of total solids and nutrients while depressing dissolved oxygen concentrations. Due to the distance from the river mouth, these conditions do not occur as frequently at the Siuslaw River in Mapleton as they do at the Klaskanine River site. These conditions cause the summer average OWQI score to be lower than the rest of the year (Table 1).

Table 1. Seasonal Average OWQI Results for the Mid Coast Basin (WY 1986 -1995)

Summer: June - September; FWS ( Fall, Winter, & Spring):October - May
Scores - Very Poor: 0-59, Poor: 60-79, Fair: 80-84, Good: 85-89, Excellent: 90-100

Written by Curtis Cude, Oregon Department of Environmental Quality, Laboratory Division