Publication Summary

Significant decreasing linear trends in nitrate/nitrite concentration were detected at five of the six long-term sampling stations in the Snohomish River basin.

Significant decreasing linear trends in nitrate/nitrite concentration were detected at five of the six long-term sampling stations in the Snohomish River basin.

One station, the Snoqualmie River at Snoqualmie, demonstrated a seasonal increasing trend in nitrate/nitrite concentration during the low flow months (July-October). Decreasing trends in dissolved oxygen concentration were detected at two stations on the Snoqualmie River (at Carnation and Snoqualmie), but values are still above state regulatory standards. A substantial increase in the flux of nitrate, total phosphorus, and ammonia seemed to occur above the Snohomish River station which was not accounted for by the long-term measured inputs from the Pilchuck River at Snohomish, the Skykomish River at Monroe, and the Snoqualmie River at Carnation. The increase in both total phosphorus and ammonia seemed to be independent of flow and may represent a relatively constant input. The increase in the flux of nitrate and ammonia was positively related to flow, suggesting runoff or increased ungauged tributary contributions. The Wateryear 1992 data support the conclusion of a flow related increase in nitrate flux over this river reach, but do not directly support the conclusions for the other parameters. These differences are probably due to the small sample size and to natural interannual variability.