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

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Data Sources

The historical water-quality and streamflow data used in this study were obtained from the USGS and other Federal, State, and local agencies. The two main types of data were (1) results of discrete intermittent collections of nutrient and SS samples from river and stream locations in the Columbia River and Puget Sound Basins in Oregon, Washington, and Idaho, and (2) continuous measurements of streamflow at locations where the water-quality samples were collected or from nearby streamflow stations. Combining data from these agencies was important because no single agency had data of sufficient quantity or quality to be used alone for the analysis (Hirsch and others, 2006). To aid in explaining the trends in nutrients and sediment, and the spatial patterns in yields, ancillary information on individual stream catchments also was compiled (Hirsch and others, 2006). This ancillary information included basin climate, hydrology, drainage area, land cover, population and road densities, National Resources Inventory of agricultural management practices, ecoregions, nutrient ecoregions, hydrologic landscapes, physiographic divisions, bedrock and surficial geology, as well as catchment-specific estimates for atmospheric deposition, fertilizer, and manure loads (provided to the study by the USGS NAWQA Pesticide Synthesis Workgroup [http://ca.water.usgs.gov/pnsp/]). A technique described by Nakagaki and Wolock (2005) was used to reapportion the county-level ancillary data to the individual drainage areas upstream of the water-quality site used in the analysis.

Discrete Water-Quality Samples

Of the 100 water-quality sites used in our analysis, 36 sites were operated by the USGS (fig. 1, table 1). Figure 1 shows the NAWQA study areas in the Columbia River and Puget Sound Basins (the USGS sites considered for this analysis were not restricted to these areas). As part of the USGS program of disseminating water data to the public, a distributed network of computers and fileservers is maintained for the storage and retrieval of water data collected through its activities at about 1.4 million sites. This system is called the National Water Information System (NWIS). Many types of data are stored in NWIS, including site information, time-series (stage, streamflow, precipitation, and chemical), peak flow, ground water levels, water-quality data, and water-use data (U.S. Geological Survey, 1998). The USGS uses the 5-digit parameter code system developed by the U.S. Environmental Protection Agency (USEPA) in the 1960s for its STORET (STOrage and RETrieval) system. The parameter codes (Pcodes) are used to identify each measured constituent in the record, defined by a set of attributes.

Additional water-quality data were retrieved from monitoring sites operated by various local, State, and Federal environmental quality agencies. Data from 26 sites were obtained from the Washington State Department of Ecology (WDOE) through their River and Stream Water-Quality Network database (Washington State Department of Ecology, 2004). The goal of the WDOE network is to provide timely water-quality data for various purposes, including evaluation of the status of and trends in stream water quality statewide. Data from another 23 water-quality sites were obtained from the Pacific Northwest Water-Quality Exchange (Exchange) and from the Oregon Department of Environmental Quality (Gregory Pettit, Oregon Department of Environmental Quality, written commun., 2006). The Exchange comprises a number of related information management projects that collectively seek to facilitate the aggregation of and access to a comprehensive source of data related to water quality in the Pacific Northwest. (Pacific Northwest Water Quality Exchange, 2006). All 23 sites were operated by the Oregon Department of Environmental Quality (ODEQ). Data from an additional 13 sites were retrieved from a database maintained by Clean Water Services (CWS) (Janice Miller, Clean Water Services, written commun., 2005). CWS provides wastewater treatment and management of surface-water resources for the 712 mi² Tualatin River basin, in northwestern Oregon (Clean Water Services, 2006). Data from the remaining two sites were retrieved from the USEPA STORET data management system (U.S. Environmental Protection Agency, 2006b). These sites were operated by the Bureau of Reclamation in eastern Oregon (table 1).

Daily Mean Streamflow

The 100 daily mean streamflow files were obtained from several data sources: (1) 80 streamflow files were retrieved from the USGS NWIS database; (2) 7 streamflow files from the Oregon Water Resources Division (OWRD) database (Oregon Water Resources Division, 2006); (3) 10 streamflow files from the Washington County, Oreg., Watermaster (Washington County, Oregon, 2006); and (4) 3 streamflow files from the Bureau of Reclamation Hydromet database (Bureau of Reclamation, 2006).

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