The concentrations of dissolved solids, chloride, and sulfate in Livingston Reservoir on the Trinity River in southeastern Texas usually average less than 250 mg/l (milligrams per liter), 40 mg/l, and 50 mg/l, respectively. The water is usually hard or moderately hard (61 to 180 mg/l as calcium carbonate). The concentrations of principal dissolved constituents in the reservoir are usually maximum during summer and fall when evaporation is high and inflow is low. Thermal stratification of the reservoir usually begins in March and persists until September or October. Neither the seasonal variation of dissolved constituents in inflow to the reservoir nor thermal stratification has resulted in significant stratification of the principal dissolved constituents. However, thermal stratification has resulted in significant seasonal and areal variations of dissolved oxygen, which results in higher concentration of dissolved iron, dissolved manganese, total phosphorus, and total inorganic nitrogen. Oxygen utilized in the stabilization of unoxidized material from upstream sources, decaying algae, and pre-existing organic material along the bottom of the reservoir is not replaced during periods of summer stagnation; and water below depths of 25 to 35 feet (8 to 11 meters) usually contains less than 1.0 mg/l dissolved oxygen. During periods of summer stagnation, reducing conditions often result in the solution of iron and manganese from bottom sediments in the deep parts of the reservoir. At site AC, a deep site near Livingston Dam, dissolved-iron concentrations in water near the bottom of the reservoir during summer have ranged from 80 to 2,300 μg/l (micrograms per liter) and have averaged about 750 μg/l. The concentrations of dissolved manganese in water near the bottom of the reservoir at this site during summer have ranged from 230 to 4,700 μg/l and have averaged about 2,600 μg/l. Water near the surface of the reservoir throughout the year and water near the bottom during periods of winter circulation usually contain less than 100 μg/l of dissolved iron and 100 μg/l of dissolved manganese. The concentrations of total phosphorus and inorganic nitrogen in water near the bottom at deep sites near Livingston Dam are usually maximum during periods of summer stagnation when decay of aquatic organisms and chemical reduction of bottom sediments release phosphorus and nitrogen to the water. The concentrations of phosphorus in the bottom stratum of water at site AC average about 2.0 mg/l. The concentrations of inorganic nitrogen in the bottom and surface strata at this site during summer average about 4.0 mg/l and 0.1 mg/l, respectively. Seasonal temperature and dissolved oxygen cycles have resulted in significant quantities of dissolved iron, dissolved manganese, total phosphorus, and total inorganic nitrogen being trapped and recycled within the reservoir.