Two water-quality synoptic studies were made on the Colorado River in the Grand Canyon, Arizona. Field measurements and the collection of water samples for laboratory analysis were made at 10 mainstem and 6 tributary sites every 6 hours for a 48-hour period on November 5-6, 1990, and again on June 18-20, 1991. Field measurements included discharge, alkalinity, water temperature, light penetration, pH, specific conductance, and dissolved oxygen. Water samples were collected for the laboratory analysis of major and minor ions (calcium, magnesium, sodium, potassium, strontium, chloride, sulfate, silica as SiO 2), trace elements (aluminum, arsenic, boron, barium, beryllium, cadmium, cobalt, chromium, copper, iron, lead, lithium, manganese, molybdenum, nickel, selenium, thallium, uranium, vanadium and zinc), and nutrients (phosphate, nitrate, ammonium, nitrite, total dissolved nitrogen, total dissolved phosphorus and dissolved organic carbon). Biological measurements included drift (benthic invertebrates and detrital material), and benthic invertebrates from the river bottom. Laboratory Several laboratory instrumental methods were used for the analysis of samples collected during the synoptic studies. Techniques were selected based on their optimal use for specific measurement criteria, primarily accuracy, precision, sensitivity, and selectivity. These techniques have been developed over a period of several years for the specific application to environmental water analysis. Alkalinity was determined using an automated Gran titration method (Kramer, 1982). Titrations were performed on a Radiometer auto-titrator using the supernatant portions of the sample after letting the sample settle for at least 24 hours and titrated with a 0.1 N standard sulfuric acid solution. Major cations were determined using inductively coupled plasma-atomic emission spectrometric (ICP-AES) analysis. Specific details of the method and techniques are described in Garbarino and Taylor (1979). The elements calcium, iron, magnesium, silica (as SiO 2), sodium, and strontium were determined simultaneously on a sample aliquot that had been filtered (0.4-µm) and acidified (HNO 3) in the field, using ultraclean procedures. Potassium was determined on the same aliquot by an air/acetylene flame atomic absorption spectrometric technique (Fishman and Friedman, 1985).