Thermal stratification in Lake Conroe usually begins to develop in March and persists until October. Thermal stratification has resulted in significant seasonal and area! 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 were less than 120 milligrams per liter, those of dissolved chloride generally were less than 22 milligrams per liter, and those of dissolved sulfate were less than 10 milligrams per liter 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 greater than 60 but less than 120 milligrams per liter as calcium carbonate). The average concentrations of dissolved oxygen at most sites in the downstream one-half of the lake averaged 3.2 milligrams per liter during summer stratification and more than 9 milligrams per liter during winter circulation. The concentrations at most sites in the headwaters of the lake averaged less than 4.3 milligrams per liter during the summer and less than 7.9 milligrams per liter during the winter. Water below depths of 25 to 35 feet usually contained less than 1 milligram per liter 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 less than 100 micrograms per liter. The greatest concentration occurred during summer stagnation near the reservoir bottom at site Ac , 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. I2 13 concentrations of dissolved winter surveys.

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Somerville Lake in south-central Texas is a shallow lake, with a mean depth of 14 feet. The maximum depth of the submerged channel of Yegua Creek is usually less than 35 feet and in most areas of the lake the depth is less than 10 feet. Several factors including thermal circulation resulting from the cooling of surface water, wind action, and the large inflow volume in realtion to the lake volume combine to keep the lake well mixed throughout the year. The oxygen concentrations remain high areally and at depth because of good circulation of lake waters during most of the year. Even in summer most bottom oxygen concentrations were in excess of 50 percent of saturation. the water ranged from 75 to 140 milligrams per liter, expressed as calcium carbonate, placing it in the moderately hard to hard (61 to 180 milligrams per liter) classification. The concentrations of principal dissolved constituents indicate that Somerville Lake is an excellent source of water for municipal, industrial, or agricultural use. I en 01 DISCHARGE, IN CUBIC METERS PER SECOND

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