Conclusions


Results from the TN and TP models showed that anthropogenic nutrient sources were responsible for a substantial part of the nutrient load in some reaches. The models also showed, however, that the largest average contribution to TN and TP load in PNW streams was from natural nutrient sources and that most of the contribution from anthropogenic nutrient sources was from diffuse rather than point sources. This means that regulatory actions taken to manage nutrient loads should consider the importance of natural sources and diffuse anthropogenic sources such as atmospheric deposition, farm fertilizer, and grazing livestock as well as point sources. Finally, although urban sources were the largest local source of TN and TP in only a small percentage of the incremental catchments, the concentration of these catchments around Portland and the Puget Sound indicated that anthropogenic urban sources in these highly developed areas were responsible for a substantial amount of the nutrients in surface water.


The findings from this study could help complement research and inform water-quality management in the PNW. Two examples were presented that showed how the SPARROW model can be used to assess nutrient conditions in large rivers that drain watersheds with a mix of land cover. The Willamette Ecosystem Services Project being developed by the U.S. Environmental Protection Agency will examine how land use and human activities affect the distribution of reactive nitrogen in the Willamette Basin (Compton and others, 2009). The results from the SPARROW TN model described in this report could complement the Willamette Ecosystem Services Project by showing the distribution of nitrogen in surface waters within the Willamette Basin. The approach used to analyze TN loads and sources in the Willamette River also could be used as a template for estimating the contribution from different sources to the nitrogen load in tributary watersheds. The States of Idaho and Oregon have designated most of the Snake River below Twin Falls as water-quality impaired and the resulting Total Maximum Daily Loads (TMDLs) are focused on reducing phosphorus inputs to this stretch of the river. The results of this study provided a complete description of the TP load throughout the Snake River Basin and the relative contribution from all major phosphorus sources to that load, information that should be useful when TMDLs are developed or refined in the future. In addition to the model results, researchers and water-quality managers might find value in the input data sets used in this study. These data sets are the most comprehensive collection of surface-water nutrient measurements and landscape nutrient loading estimates compiled to date for the PNW.


First posted July 17, 2013