The water quality of Sam Rayburn Reservoir, eastern Texas

Abstract

Inflow of wastes to the Angelina River has caused some local deterioration of the quality of water downstream from Lufkin. However, the volume of flow in the Angelina River has been adequate to prevent serious deterioration of the quality of water in Sam Rayburn Reservoir. From March 1965 to September 1968, the time-weighted concentration of dissolved solids in water released from Sam Rayburn Reservoir averaged about 120 mg/l (milligrams per liter). The average dissolved-solids content of water in the reservoir during 13 surveys ranged from about 100 to 145 mg/l. The dissolved-oxygen content of water in the reservoir varied seasonally and was intimately related 'to the pattern of thermal stratification. During 10 reservoir surveys, the depth-integrated concentration of dissolved oxygen at deep sites in the downstream half of the reservoir averaged more than 5 mg/l. The concentration of dissolved oxygen usually was much greater during periods of winter circulation than during periods of summer stagnation. About 2 river miles upstream from Sam Rayburn Dam, the depth-integrated dissolved-oxygen concentration ranged from 1.2 mg/l (16-percent saturation) on June 30, 1965., to 10.9 mg/1 (91-percent saturation) on February 4, 1966. During periods of summer stagnation, water below depths of 25-35 feet usually contained less than 2.5 mg/l dissolved oxygen and often contained less than 1.0 mg/l. The dissolved-oxygen content of water usually was less in the upstream half of the reservoir than in the downstream half. During 10 reservoir surveys, the depth-integrated concentration of dissolved oxygen about 41.5 miles upstream from Sam Rayburn Dam averaged 4.2 mg/l. Part of the dissolved-oxygen deficit (difference between saturated concentration and actual concentration) resulted from the inflow of wastes; however, data for tributary arms of the reservoir indicate that part of the dissolved-oxygen deficit resulted from the decomposition of naturally occurring organic debris in the water and in the area inundated by the reservoir. Concentrations of iron and manganese in the water varied seasonally and were related to the dissolved-oxygen content of the water. The concentrations of iron and manganese throughout the reservoir were much smaller during periods of winter circulation than during periods of summer stagnation. During each of three reservoir surveys in February, the concentrations of iron and manganese near Sam Rayburn Dam were less than 0.40 and 0.25 mg/l, respectively. However, on October 6, 1965, the iron content of water ranged from less than 1 mg/l at depths less than 30 feet to as much at 14 mg/l at greater depths. Similarly, on September 9, 1966, the concentration of manganese near the dam ranged from less than 0.5 mg/l at depths less than 10 feet to as much as 6.9 mg/l at greater depths. Although storage of water in Sam Rayburn Reservoir has resulted in a decrease in variations of dissolved solids and principal chemical constituents in the Angelina River downstream from the reservoir, it has resulted in significant seasonal variations in ,the concentrations of dissolved oxygen, iron, and manganese at downstream sites. Results of periodic surveys indicate that dissolved-oxygen concentrations at three sites in the 19-mile reach of the Angelina River downstream from Sam Rayburn Dam were low in late summer and early fall after periods of summer stagnation in the reservoir. Moreover, the amount of reaeration that occurred in the reach was insignificant. During periods when the dissolved-oxygen deficiency was large, the concentrations of iron and manganese at each of the three sites increased greatly.