Water-quality of Lake Conroe on the West Fork San Jacinto River, southeastern Texas
Thermal stratification in Lake Conroe, Texas usually begins to develop in March and persists until October. Thermal stratification has resulted in significant seasonal and areal variations in the concentrations of dissolved oxygen, dissolved iron, dissolved manganese, total inorganic nitrogen, and total phosphorus. Volume-weighted-average concentration of dissolved solids generally was < 120 mg/L, that of dissolved chloride generally was < 22 mg/L, and that of dissolved sulfate was < 10 mg/L in Lake Conroe during the 1973-82 water years. The concentrations of each of these constituents usually were largest during the summer. The water was moderately hard (hardness > 60 but < 120 mg/L as calcium carbonate). The average concentrations of dissolved oxygen at most sites in the downstream one-half of the lake averaged 3.2 mg/L during summer stratification and > 9 mg/L during winter circulation. The concentrations at most sites in the headwaters of the lake averaged < 4.3 mg/L during the summer and < 7.9 mg/L during the winter. Water below depths of 25 to 35 ft usually contained < 1 mg/L dissolved oxygen during the summer. The concentrations of dissolved iron and dissolved manganese in water throughout the reservoir during winter circulation and in water near the reservoir surface during summer stratification were < 100 micrograms/L. The greatest concentration occurred during summer stagnation near the reservoir bottom at a deep site near Lake Conroe Dam. The concentrations of total inorganic nitrogen and total phosphorus were greatest during summer stratification in water near the reservoir bottom at deep sites. No accumulation of these constituents within the reservoir was detected during the study. The densities and composition of algal populations varied seasonally. Algal densities were greatest during the summer with blue-green algae being the predominant phylum.