U.S. Geological Survey U.S. Geological Survey
Frank A. Rinella, U.S. Geological Survey, 10615 S.E. Cherry Blossom Drive, Portland, Oregon 97216
Dennis A. Wentz, USGS, Portland, Oregon
Kathryn M. Kuivila, USGS, Sacramento, California
The Pudding River Basin encompasses 1400 square kilometers of the Willamette River Basin and is Oregon's leading agricultural region in terms of gross sales. Stream-water samples were collected in the Pudding River at Aurora during high- and low-flow conditions from April through November as part of the Willamette National Water-Quality Assessment (NAWQA) Study. Median concentrations of atrazine and simazine were more than 3 times larger at high flows than at low flows. The median concentration of nitrite plus nitrate was about the same during both flow conditions, whereas the median concentration of soluble reactive phosphorus was three times larger at the low flows. These data suggest that different sources or processes are influencing transport of these constituents to the stream under different flow conditions. Instantaneous loads for atrazine and simazine averaged 13.5 to 15 times larger and those for nitrate were 9.5 times larger at high flows than at low flows. The instantaneous load for soluble reactive phosphorus was still nearly 3 times larger at the high flows, despite its lower concentration at the high flows. Thus, the largest loads for these dissolved constituents were transported from the basin during the high streamflows.