Hydrologic Accounting Unit 120301 is located in north-central Texas and includes that part of the Trinity River basin upstream from the National Stream Quality Accounting Network station 08062700, Trinity River at Trinidad, Texas. Normal annual precipitation ranges from about 28 inches in the western part of the basin to almost 40 inches in the eastern part. The population of the upper Trinity River basin was nearly 3 million in 1980. Total water use in 1980 was approximately 826,000 acre-feet, of which 90 percent was obtained from surface-water resources.

Mean discharges for most stations in the study area during the 1973-82 water years were significantly larger than mean discharges for the period of record. Mean discharges throughout most of the study area during the 1982 water year were more than 300 percent of the long-term mean flow.

Water in the upper Trinity River basin upstream of the Dallas-Fort Worth metroplex is generally a calcium bicarbonate type water. In the West Fork Trinity River downstream from Fort Worth and the main stem of the Trinity River from Dallas to Trinidad, the water is a mixed sodium calcium bicarbonate type water. Average dissolved-solids concentrations during the study period ranged from 174 milligrams per liter in the Elm Fork Trinity River near Lewisville to 682 milligrams per liter in Mountain Creek near Cedar Hill. Discharge-weighted average dissolved-solids concentrations during 1973-82 water years ranged from 165 milligrams per liter in the Elm Fork Trinity River near Lewisville to 416 milligrams per liter at Mountain Creek near Cedar Hill.

Daily dissolved-solids loads increased from 225 tons at West Fork Trinity River at Beach Street, Fort Worth, to 2,410 tons in the Trinity River at Trinidad. In the 450 square miles of intervening drainage area between West Fork Trinity River at Beach Street, Fort Worth, and West Fork Trinity River at Grand Prairie, the average daily dissolved-solids load increased by 305 tons.

Upstream of the municipal waste effluents in the Dallas-Fort Worth metroplex, average dissolved-oxygen concentrations generally exceed 7.0 milligrams per liter, average dissolved-oxygen percent-saturation values generally exceed 70 percent, and average biochemical oxygen demand concentrations generally were less than 4.0 milligrams per liter. Downstream of the municipal waste effluents, average dissolved-oxygen concentrations generally did not exceed 5.0 milligrams per liter, average dissolved-oxygen percent-saturation values generally were less than 50 percent, and average biochemical oxygen demand concentrations generally were greater than 12.0 milligrams per liter.