Water quality of Canyon Lake, central Texas

Abstract

The volume-weighted average concentrations of the principal dissolved constituents in Canyon Lake on the Guadalupe River in central Texas are usually less than 240 milligrams per liter of dissolved solids, 20 milligrams per liter of chloride, and 30 milligrams per liter of sulfate. The water, which is very hard, has a volume-weighted average concentration of hardness of about 200 milligrams per liter. There is little seasonal variation in the volume-weighted average concentrations of the principal dissolved constituents. Thermal stratification of the lake usually begins during March and persists until September or October. Stratification results in significant seasonal and areal variations in dissolved oxygen, which in turn result in higher concentrations of dissolved iron, dissolved manganese, and total ammonia during the summer. Oxygen used in the stabilization of unoxidized material in the lake is not replaced during summer stagnation. The depth-integrated concentration of dissolved oxygen averaged less than 4.0 milligrams per liter during summer stagnation and about 9.0 milligrams per liter during winter circulation. The concentrations of dissolved iron and dissolved manganese, which varied seasonally, were closely related to the concentrations of dissolved oxygen. Reducing conditions in the hypolimnion often result in the dissolution of iron and manganese from bottom sediments in the deep parts of the lake. At site Dc, a deep site on an arm of Canyon Lake, the summer concentrations of dissolved iron averaged 860 micrograms per liter and the concentration of dissolved manganese averaged 390 micrograms per liter. The concentrations of total ammonia in the lake usually were less than 0.2 milligram per liter except in the hypolimnion (bottom stratum) during summer stagnation when nitrate and nitrite are reduced to ammonia. The closure of Canyon Dam resulted in a change in the monthly average water temperature of the Guadalupe River downstream from the dam. Prior to closure, the maximum monthly average water temperature for the Guadalupe River near Sattler, which was 29.0? degrees Celsius, occurred during June or July. Since closure of the dam, the maximum monthly average water temperature, which is 19.0? degrees Celsius, occurs during September or November.