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GLOSSARY Basic Fixed Sites-Sites on streams at which streamflow is measured and samples are collected for temperature, salinity, suspended sediment, major ions and metals, nutrients, and organic carbon to assess the broad-scale spatial and temporal character and transport of inorganic constituents of stream water in relation to hydrologic conditions and environmental settings. Bed-Sediment and Tissue Studies-Assessment of concentrations and distributions of trace elements and hydrophobic organic contaminants in stream bed sediment and tissues of aquatic organisms to identify potential sources and assess spatial distribution. Depth-integrating sampler-A sampler that will integrate and represent the area of a stream section. Discharge-weighted samplers-A sampler that will isokinetically represent the stream flow. Ecological Studies-Studies of biological communities habitat characteristics to evaluate the effects of physical and chemical characteristics of water and hydrologic conditions on aquatic biota and to determine how biological and habitat characteristics differ among environmental settings in Study Units. Equal-width increment (EWI) sampling-A composite sample across a section of stream with equal spacing between verticals and equal transit rates within each vertical that yields a representative sample of stream conditions. Gaging station-A fixed site on a stream or river where hydrologic and environmental data are collected. Indicator Sites-Stream sampling sites located at outlets of drainage basins with relatively homogeneous land use and physiographic conditions. Basins are as large and representative as possible, but still encompassing primarily one Environmental Setting (typically, 50 to 500 km2). Integrator Site-Stream sampling sites located downstream of drainage basins that are large and complex and often contain multiple Environmental Settings. Most Integrator Sites are on major streams with drainage basins that include a substantial portion of the Study Unit area (typically, 10 to 100 percent). Intensive Fixed Sites-Basic Fixed Sites with increased sampling frequency during selected seasonal periods and analysis of dissolved pesticides for 1 year. Most Study Units have one or two integrator Intensive Fixed Sites and one to four indicator Intensive Fixed Sites. Isokinetic sampling-The water entering the sampler is hydrodynamically equivalent (velocity, area, and direction) to the portion of the stream being sampled. Occurrence and Distribution Assessment-Assessment of the broad-scale geographic and seasonal distributions of water-quality conditions for surface and ground water of a Study Unit in relation to major contaminant sources and background conditions. Solid-phase extraction (SPE)-A procedure to isolate specific organic compounds onto a bonded silica extraction column. Study Unit-A major hydrologic system of the United States in which NA WQA studies are focused. NA WQA Study Units are geographically defined b~ a combination of ground- and surface-water features and usually encompass more than 10,000 km of land area. The NA WQA design is based on assessment of 60 Study Units, which collectively cover a large part of the Nation, encompass the majority of population and water use, and include diverse hydrologic systems that differ widely in natural and human factors that affect water quality. Water-Column Studies-Assessment of physical and chemical characteristics of stream water, including suspended sediment, dissolved solids, major ions and metals, nutrients, organic carbon, and dissolved pesticides, in relation to hydrologic conditions, sources, and transport. Glossary VII