By Valerie J. Kelly, Richard P. Hooper, Brent T. Aulenbach, and Mary Janet
Monitoring the water quality of rivers in the United States has been an important part of the mission of the U.S. Geological Survey (USGS) for decades. Beginning in the 1970s, the National Stream Quality Accounting Network (NASQAN) provided nationally consistent data for as many as 500 stations sampled monthly (Ficke and Hawkinson, 1975). NASQAN data from 1974 through 1995 were contained in USGS digital Data Series 37 (Alexander and others, 1996). With the implementation of the National Water Quality Assessment (NAWQA) (Hirsch and others, 1998), NASQAN was re-designed to focus on rivers draining basins larger than those studied by NAWQA. NAWQA study "units" (most, but not all, are single river basins) encompass an average area of about 50,000 km2. Accordingly, six large-river basins in the United States were candidates initially for the NASQAN program: the Mississippi (including the Ohio and Missouri Rivers) at 3.1 million km2, St. Lawrence/Great Lakes (1.0 million km2), Rio Grande (870,000 km2), Yukon (850,000 km2), Columbia (670,000 km2) and Colorado (650,000 km2) (Kammerer, 1990). The next largest basin, the Kuskokwim River Basin in Alaska, is within the range of NAWQA study-unit areas at 124,000 km2. Because of resource limitations and logistical difficulties, the Yukon River and St. Lawrence/Great Lakes Basins were not sampled by NASQAN during the time period reported here (1996-2000).
The water quality in the four river basins selected for study was characterized by estimating the annual mass flux, or loading, of constituents at a network of fixed sites. Mass flux is the amount of material that passes a station, generally expressed as tons per day, and is determined by multiplying the concentration of a constituent by the stream discharge. This approach provides mass balances to compare inputs (e.g., amounts of agricultural chemicals applied in a drainage basin) with riverine outputs so as to identify constituent source and sink areas along the river system, and to determine loadings to receiving waters, such as reservoirs and oceans. The annual flux approach appears to provide adequate resolution to characterize relative contributions from the large areas of these river basins. This approach had been used successfully in previous USGS studies of the Mississippi River Basin (Goolsby and Battaglin, 1995). Although interpretation of flux results is more difficult in the arid and more highly regulated western basins than in the Mississippi, the same sampling and data analysis strategy was applied for national consistency for the first 5 years (1996 - 2000) of the redesigned NASQAN operation.
This report presents NASQAN data collected during the period 1996-2000, including calculated annual loads for selected constituents. The report documents the sampling sites, sampling procedures, laboratory-analysis methods, results from the quality-assurance program, and the methods for load calculation. The data are stored in data files, organized by constituent group, in American Standard Code for Information Interchange (ASCII) format, readable on DOS, UNIX, and Macintosh platforms. Because the data are provided in ASCII format, they are easily accessible for computer applications. The report is available via the world-wide web, accessible via the USGS NASQAN home page (http://water.usgs.gov/nasqan).
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