A series of seven studies of the quality and quantity of low flows in a 35.5-mile reach of the Colorado River upstream from Colorado City, Texas, were made from February 1975 to March 1978 to delineate areas of saline inflow. These studies showed generally that ground water contributed throughout the reach is saline, but that loads of dissolved constituents in ground-water accretions are highest in three subreaches. Yields per mile of river channel from these subreaches during the low-flow studies averaged more than 5.5 tons of dissolved solids per day, of which more than 1.8 tons were sodium and 2.9 tons were chloride.

Salt-load trend studies for three long-term continuous streamflow and daily water-quality stations show that the salinity of the flow upstream from Ira (mile 826.3) increased significantly after 1963 but decreased significantly after 1970. Part of the reach upstream from Ira is proximate to oil fields. The production and open-pit disposal of oil-field brines in the area increased significantly in the early 1960's; but a ban on open-pit disposal was enacted in 1969. No significant downward trend of salinity in flow at other daily water-quality stations downstream from Ira occurred after the ban on open-pit disposal of oilfield brines.

The low-flow and salt-load trend studies indicate that part of the salinity in the flow of the Colorado River has resulted from the inflow of oil-field brine; but preponderant evidence indicates that the major part of the salinity is of natural origin. Neither the ban on open-pit disposal nor pumping of saline ground water has significantly reduced the salinity of flow downstream from Cuthbert (mile 810.6).

Diversion of saline low flows from the Colorado River at mile 799.3 upstream from Colorado City since January 1969 has resulted in significant improvement in the quality of water. Decreases in the dischargeweighted averages of dissolved solids and chloride in the flow of the Colorado River at Colorado City (mile 796.3) during the 1969-78 water years were about 420 milligrams per liter and 280 milligrams per liter, respectively.