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U.S. GEOLOGICAL SURVEY
Scientific Investigations Report 2007–5186

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Summary and Conclusions

In 1991, the U.S. Geological Survey (USGS) implemented the National Water-Quality Assessment (NAWQA) Program to support national, regional, State, and local information needs and decisions related to water-quality management and policy (http://water.usgs.gov/nawqa). NAWQA was designed to answer the following types of questions: What is the condition of our streams? What are the factors (contaminant sources, loads, and yields) affecting the conditions? Is the water quality changing over time? From 1991 to 2001, the NAWQA Program completed interdisciplinary assessments and established a baseline understanding of water-quality conditions in 51 of the Nation’s river basins and aquifers. In the Pacific Northwest, the NAWQA Program has collected long-term water-quality data for the Columbia River and Puget Sound Basins in Oregon, Washington, and Idaho. In addition, other Federal, State, and local agencies also have collected water-quality data in these areas. For this study, multiagency data were compiled and used to provide insights into the recent status of water-quality conditions in the Pacific Northwest; how these conditions have changed over time; and how natural features and (or) human activities may have contributed to these changes.

This study examined three topics that might be of interest to water-resource managers in the Pacific Northwest: (1) Annual loads of total nitrogen (TN), total phosphorus (TP), and suspended sediment (SS) transported through the Columbia River and Puget Sound Basins were estimated for water years 1997 (high-flow), 2000 (average-flow), and 2001 (low-flow); (2) Annual yields (loads divided by catchment areas) of TN, TP, and SS for water year 2000 were compared to differences in landscape and climatic conditions between sub-basin catchments; and (3) Trends in TN, TP, and SS concentrations and loads were calculated for WY 1993–2003 and compared to changes in landscape and climatic conditions in the catchments. Trends in concentrations were determined in two ways: (1) flow-adjusted trend in concentration (FATC) was used to determine water-quality trends in the absence of trends in natural streamflow and (2) non-flow adjusted trend in concentration (NFATC) was used to express the overall trend of exposure to biota resulting from natural and anthropogenic changes. Depending on the constituent, annual loads were estimated for as many as 88 water-quality sites; annual yields were estimated for as many as 81 catchments; and trends were estimated for as many as 50 water-quality sites. The catchments ranged in size from 5 km² to more than 650,000 km², with most sites between 100 and 10,000 km². Although the catchments tended to have large percentages of forested and undeveloped land and small percentages of agricultural and urban land, there were a few catchments where agricultural and/or urban land made up most of the watershed.

Transport of Nutrients and Suspended-Sediment

During WY 2000, total point-source nutrient loads to the catchments generally were equal to a small percentage of the estimated in-stream loads. In 10 percent of the catchments, annual point-source TN loads were greater than 25 percent of the estimated annual in-stream TN load discharged from the catchment, and in 22 percent of the catchments, annual point-source TP loads were greater than 25 percent of the estimated annual in-stream TP load discharged from the catchment. Point-source nutrient loads generally also were a small percentage of the total nutrient load to the catchments compared to nonpoint sources, such as fertilizer and manure and atmospheric deposition, and loads from sources other than municipal wastewater treatment plants were never more than 5 percent of the total point-source load. However, there were a few examples where point sources comprised as much as 30 percent of the TN load and as much as 80 percent of the TP load to a catchment. In smaller urbanized catchments, point sources tended to contribute more of the total nutrient load when compared to larger catchments with less urbanization. Although point sources contributed very little to the in-stream load of TN and TP in the eight tributaries delivering most of the nutrients to the Puget Sound, the nitrogen and phosphorus loads from point sources discharging directly to the Puget Sound were about equal to the annual loads from those tributaries.

The annual mean load discharged from the Columbia River to the Pacific Ocean was about 570,000 pounds per day of TN, about 55,000 pounds per day of TP, and about 14,000 tons per day of SS. The monitored tributaries contributing the most to the annual nutrient loads in the Columbia River were the Yakima, Snake, Deschutes, and Willamette Rivers (in downstream order). Total annual loads of TN and TP from these tributaries were equal to about 63 percent of the load near the mouth of the Columbia River, whereas the annual SS load from these four tributaries was equal to about 32 percent of the load near the mouth. The estimated annual loads from point sources discharging directly into the Columbia River were equal to less than 10 percent of the annual TN and TP loads near the mouth. Generally, direct point-source discharges of nutrients to the Willamette and Snake Rivers were relatively small when compared to the total nutrient loads these rivers discharged to the Columbia. However, in some reaches of the Willamette River, point sources were responsible for much of the gain in phosphorus load.

The yields of TN, TP, and SS were generally greater in catchments west of the Cascade Range, where precipitation and runoff were greater. A multiple linear regression analysis was performed to determine the relative importance of specific catchment characteristics on yields. Based on the standardized regression coefficients, TN yields were significantly (p< 0.05) and positively related to precipitation, atmospheric nitrogen loads, fertilizer and manure loads, point-source loads, and were negatively related to average slope. TP yields were significantly and positively related to precipitation and point-source loads, and SS yields were significantly and positively related to precipitation.

Trends in Nutrients and Suspended-Sediment, 1993–2003

For most catchments, the net change in non-hydrologic characteristics (land use and other human activities) was not great enough to cause any significant (p< 0.05) flow-adjusted trend in concentration (FATC) for TN (52 percent), TP (68 percent), or SS (60 percent) between WY 1993–2003. However, some non-hydrologic changes might still have occurred in these catchments, but either they cancelled out (for example, decreasing non-point source loads and increasing point-source loads) or were too far upstream to have much effect on water quality. Twenty-one of the 44 sites available for TN trend analysis had significant FATC for TN (2 increasing, 19 decreasing). Ten sites showed significant trend in load for TN (all decreasing), and more than 70 percent of the sites had decreasing (but not necessarily significant) FATC and trend in load for TN. Sixteen of the 50 sites available for TP trend analysis had significant FATC for TP (7 increasing and 9 decreasing). Six sites had significant trend in load for TP (3 increasing, 3 decreasing), and about 50 percent of the sites had decreasing (but not necessarily significant) FATC and significant trend in load for TP. Nineteen of the 48 sites available for SS trend analysis had significant FATC for SS (4 increasing, 15 decreasing). Seven sites showed significant trend in load for SS (1 increasing, 6 decreasing), and more than 65 percent of the sites had decreasing (but not necessarily significant) FATC and trend in load for SS. Streamflow at only two of the 50 trend sites had significantly decreasing trends, indicating that almost all trends in load resulted from FATC. The values for NFATC for TN, TP, and SS were similar in magnitude to the values for FATC.

The results from this study indicate that nonpoint sources of nutrients probably have decreased over time in many of the catchments. For the Willamette River Basin, some data supported this hypothesis (total annual flow and TP loads from the major WWTPs in the basin increased between 1991 and 2000). Despite the generally small contribution of point-source nutrient loads, point sources still may have been partially responsible for the significant decreasing FATC for TN at the six sites where the total point-source nutrient loads to the catchments exceeded 25 percent of the in-stream load. Although the decreasing FATC for TP at three lower Willamette River sites could not be explained by the available data on phosphorus load from fertilizer and manure, the limited data on point sources showing large increases in TP loads to the basin between 1991 and 2000 indicate that an overall decrease in nonpoint-source phosphorus load may have been the cause.

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