The U.S. Geological Survey, in cooperation with the North Central Texas Council of Governments, the Trinity River Authority of Texas, and the Texas Water Development Board, conducted two time-of-travel studies in the Trinity River basin in November and December, 1975. Field data were collected on the East Fork Trinity River during November 18-22, 1975, and on the Elm Fork Trinity River during December 8-13, 1975. The purpose of these two studies was to provide data that could be used by the Trinity River Authority and the Texas Water Quality Board in the development of a mathematical water-quality model of the two streams. The model is to be used in a comprehensive water-quality management plan for the Trinity River basin.