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{)3~S. The chemical constituents presented in this rcport were analyzed at the U.S. Geological Survey National Water Quality Laboratory (NWQL) in Arvada., Colo., using methods listed in Fishman and others (1994). The inorganic constituents, including dissolved solids, major nutrients, and trace elcments, were analyzed using mcthods described by Fishman and Friedman (1989) and Fishman (/993). Pesticides were periodically collected at five surface-watcr . stations (table I) and were analyzed using methods described by Wershaw and others (1987) and Fishman (1993). Radiochemical constituents were collected at eight surface-water stations (table I) and werc ana- lyzed using methods developed by Thatcher and others (1977). The nitrogen data presented in the "Water- Quality Data" section at the back of this rcport indicate a change in the reporting of total nitrite plus nitratc as nitrogen and total ammonia as nitrogen aftcr January I, 1993. Following a thorough laboratory evaluation of the analytical method used to generate data for the above constituents on unfiltered and filtered samples, the NWQL concluded that "no valid ba~is exists for distinguishing between unfi Itercd and filtered determinations of nutrient species" using the current (1992) method, and "concentrations for the idcntified nutricnt species arc statistically indistin- guishable" (U.S. Geological Survcy, written cornmun.. 1992). As a result, nutrient samples collectcd for thc analysis of total nitrite plus nitrate a~ nitrogen and total ammonia a~ nitrogen were replaced by the analyses of the dissolved fraction in 1993. The analysis of total phosphorus was not affected by this change. Suspended-sediment samples wcre periodically collected at 15 surface-water stations (tablc I) using methods described by Guy and Norman (1970). Thc samples were collected u.~ing a DH--48 (hand-held sampler) or a ~74 (cable-and-reel samplcr) depth- integrating sampler and the equal-width-increment method described by Sylvester and others (1990). Suspended-sediment samples were analyzcd at the U.S. Geological Survey Sediment Laboratory in Cheyenne, Wyo., using methods described by Guy (1969). Samples containing sufficient suspended- sediment concentration were analyzed spccifically for the pencentage of suspended sedimcnt finer than sand size (Icss than 0.062 rnm). Quality-Assurance Procedures Standard U.S. Geological Survey techniques were used during field collection and preservation of all water-quality samples to ensure that represcntative environmental samples were obtained for analysis. Standard field procedures included checking equip- ment operation and instrument calibrations before collecting onsite measurements. In addition to stan- dard field procedures, specific quality<ontrol proce- durcs for the collection, treatment, and analysis of water samples were followed (Sylvester and others, 1990). The quality-assurance data presented in this report include laboratory analyses of sourcc-solution blanks and field-equipment blanks. Sounce-solution blanks are samples obtained from the dcionized water sopply that is considered free of analytes of interest and that was used for rinsing and to develop field- equipment blanks. Ficld-equipment blanks consisted of deionized water that was passed over all thc surfaces of decontaminatcd sampling and processing equipment to evaluate the adequacy of field c1caning and rinsing procedures. Laboratory quality assurance includcd procedures described by Friedman and Erdmann (1982). The data generaled from quality-a~surance samples were reviewed by appropriate U.S. Gcological Survey personncl a~sociatcd with the investigation to identify potential sample contamination problems. Deficiencies in the quality-assurance data were documented and corrective actions werc taken when required. The cleaning and rinsing proccdures described in the preceding "Methods of Data Collection and Analysis" section were designed to decrease the likeli- hood of equipment-caused contamination between surface-water sites. In an effort to evaluate field- equipment cleaning procedures, a field-equipment blank was collected for laboratory analysis during each sampling trip. During the first year of sampling (1990), measured concentrations of some analytes in thc field- equipment blanks wcre higher than expccted. These results prompted a reexamination of the collection process for the field-equipment blanks. It wa~ deter- mined that deionized water that passed over somc metal equipment surfaces. which were not in contact with the cnvironmental samplc, could have biascd the results of thc field-equipment blanks. As a result, collection procedures for field-equipment blanks were modified in 1991 to better simulatc the contact that occurs between native river water and sampling- equipment surfaces during collection of the environ- mental sample. Subsequent concentrations ofanalytes determined in the quality-assurance samples were within acceptable levels throughout the remaindcr of the study. TYPES OF WATER.QUALlTY nATA 7