Lake Houston, a reservoir completed in 1954 about 25 miles east-northeast of Houston, Texas, is a principal surface-water source for the city of Houston. The increase in water supply to meet future demands is expected to be accommodated by supplementing surface-water inflows to Lake Houston. The Trinity River is considered a potential source for interbasin transfer of water to Lake Houston. Before beginning to supplement inflows, the City needs to better understand the potential effects on Lake Houston water quality from streams that flow into or might contribute water to Lake Houston. During 1983–90, the USGS collected 3,727 water-quality samples from 27 sites in Lake Houston, 6 of the 7 main tributaries to the lake, and the Trinity River at Romayor.

Longitudinal profiles of water temperature, dissolved oxygen, specific conductance, pH, and nutrients from the dam to the East and West Forks of Lake Houston constructed for a winter day and a summer day indicate that in general the lake water is mixed in the winter and stratified in the summer.

The results of Mann-Whitney rank-sum tests to determine whether there were significant differences between summer and non-summer field measurements, 5-day biological oxygen demand, bacteria, physical and aesthetic properties, nutrients, organic carbon, chlorophyll a, and trace elements in the lake nearest the dam, the East Fork of the lake, and the West Fork of the lake at the same relative depth showed significant differences between summer and non-summer samples for at least one of the three locations at the same relative depth for all 15 properties and constituents tested except specific conductance. The test results indicate that in general Lake Houston is well mixed in the non-summer period and stratified with respect to selected properties and constituents in the summer.

The results of rank-sum tests to determine whether there were significant differences between field measurements, 5-day biological oxygen demand, physical and aesthetic properties, nutrients, organic carbon, and chlorophyll a in the lake nearest the dam, the East Fork of the lake, and the West Fork of the lake for samples collected during the same season at the same relative depth showed that significant differences were common; generally, the West Fork had the largest median concentrations among the three locations. The tests comparing trace element concentrations between the lake nearest the dam and the East Fork showed mixed results—large median dissolved manganese concentrations in lake bottom samples in the summer and in East Fork near-surface samples in the non-summer period.

The results of rank-sum tests comparing selected properties, 5-day biological oxygen demand, bacteria, nutrients, and total organic carbon in the eastern tributaries with those in the western tributaries, in the eastern tributaries with those in the Trinity River, and in the western tributaries with those in the Trinity River during the same season (summer or non-summer) at the same relative streamflow (low-medium or high) showed that significant differences were more common than not. In the comparisons of the eastern tributaries with the western tributaries that resulted in significant differences, medians of the western tributaries were larger for all properties and constituents except total organic carbon; in the comparisons of the eastern tributaries with the Trinity River that resulted in significant differences, medians were larger for the Trinity River in about 60 percent of the tests; and in the comparisons of the western tributaries with the Trinity River that resulted in significant differences, medians were larger for the western tributaries in about 60 percent of the tests.

In the tests comparing trace elements between the eastern and western tributaries during the same season at the same relative streamflow, five of the eight tests showed no significant differences; between the eastern tributaries and the Trinity River, all eight tests showed significant differences, with eastern tributary medians larger in all tests; and between the western tributaries and the Trinity River, seven of the eight tests showed significant differences, with western tributary medians larger in all seven tests.

The tests comparing selected properties, 5-day biological oxygen demand, nutrients, and total organic carbon between the eastern tributaries and the East Fork of Lake Houston, between the western tributaries and the West Fork of Lake Houston, and between the Trinity River and the lake nearest the dam, the East Fork, and the West Fork during the same season (summer or nonsummer) yielded significant differences in about 60 percent of the tests. No discernible pattern emerged to associate significant differences with season.

In the tests comparing trace elements between the tributaries and the respective forks of the lake to which the tributaries drain, iron concentrations were significantly different in three of the four tests, with median concentrations larger in the tributaries. All the tests comparing manganese between the Trinity River and the three locations in the lake yielded significant differences, with larger median concentrations in the lake.