Water-quality data were collected on a 13.6-mile reach of the West Fork Trinity River in Fort Worth, Texas to test a dynamic Lagrangian model. Flow was steady. Loads of dissolved constituents varied with time at the beginning of the study reach and in the reach, primarily because of photosynthesis. River quality was fairly good despite low dissolved oxygen measured in the headwaters and the significant sewage load from the tributaries. Diel and longitudinal trends were defined by sampling at fixed sites and by following dyed parcels of water. Nitrification, deoxygenation, reaeration, and photosynthesis affected the dissolved oxygen balance. Independent estimates of some of the rate coefficients were 0.1 to 0.2, 0.8, and 0 to 3.6 , all per day, for deoxygenation, nitrification, and reaeration, respectively. (USGS)