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The Arroyo Colorado is in the lower Rio Grande Valley of southern Texas and extends from near Mission, Texas, eastward to the Laguna Madre estuarine and coastal marine system, which separates Padre Island from the Texas mainland. Streamflow in the Arroyo Colorado primarily is sustained by effluent from municipal wastewater-treatment plants along the stream banks. Since 1986, the tidal segment of the Arroyo Colorado from the port of Harlingen to Laguna Madre has been listed by the State of Texas as an impaired water body because of depressed dissolved oxygen concentrations. Efforts to develop predictive water-quality models for the tidal segment of the Arroyo Colorado have been hampered by the availability of physical, biological, and biochemical data. Specifically, data on hydrodynamics, primary algal productivity, nutrient cycling, sediment deposition rates, and the inter-relations between these processes and dissolved oxygen dynamics to support water-quality modeling efforts are lacking. The U.S. Geological Survey, in cooperation with the Texas Commission on Environmental Quality, did a study in 2006 to collect data associated with hydrodynamics, primary algal productivity, nutrient cycling, and dissolved oxygen dynamics in the tidal segment of the Arroyo Colorado. Specific objectives of the study were to (1) characterize water quality by measuring basic properties; (2) characterize the concentrations of carbon and nutrients, biochemical oxygen demand, total organic carbon, total suspended solids, and volatile suspended solids; (3) measure the seasonal differences of nutrient-dependent algal growth and algal production in the water column; (4) measure oxygen respiration or production rates; and (5) measure rates of sediment deposition.