Sigleo, A., and W. Frick. 2003. Seasonal variations in river flow and nutrient concentrations in a Northwestern USA watershed. Pages 370-375 in Proceedings, First Interagency Conference on Research in the Watersheds, Oct. 2003, Benson, AZ., US Dept of Agriculture. WED-03-115
Dissolved nutrient concentrations were measured in the Yaquina River, Oregon, from 1998 through 2001 to determine nutrient loading from the watershed as part of a larger agency program for evaluating nutrient sources. The effects of storms on dissolved nutrient transport were investigated relative to stream discharge for three storm events, including one in a high rainfall-discharge year, and two in average years, one of which followed a drought year. During the drought year (no flows >25 m3s-1), total dissolved nitrate input was considerably less than in wetter years. However, dissolved nitrate concentrations were unusually high in the first winter storm runoff after the drought. In the November 2001 storm, dissolved nitrate increased rapidly (to nearly 200 ÁM) but decreased by 20 to 30 percent as the storm progressed. The dissolved nitrate nitrogen loads varied from 17,400 kg day-1 during high-flow storm events to less than 2.25 kg day -1 during late summer, low flow conditions. Dissolved silica dynamics were quite different and during storm events silica concentrations in the Yaquina River decreased to near zero at the storm height, probably due to dilution by rapid, shallow flow, and then recovered after 48 hours. During the time interval studied, over 94% of the dissolved nitrate and silica were transported during the winter months of greater rainfall indicating that seasonality and river flow are important determinants when considering nutrient loadings.