The Lehigh River, 100 miles long, is the second largest tributary to the Delaware River. It drains 1,364 square miles in four physiographic provinces. The Lehigh River basin includes mountainous and forested areas, broad agricultural valleys and areas of urban and industrial development. In the headwaters the water is of good quality and has a low concentration of solutes. Downstream, some tributaries receive coal-mine drainage and become acidic; others drain areas underlain by limestone and acquire alkaline characteristics. The alkaline streams neutralize and dilute the acid mine water where they mix. The dissolved-oxygen content of river water, which is high in the upper reaches of the stream, is reduced in the lower reaches because of lower turbulence, higher temperature, and the respiration of organisms. The Lehigh is used for public supply, recreation, waterpower, irrigation, and mining and other industrial purposes. Because the river is shallow in its upper reaches, most of the water comes in contact with the atmosphere as it churns over rocks and around islets and large boulders. Aeration of the water is rapid. When water that was low in dissolved-oxygen concentration was released from the lower strata of the Francis E. Walter Reservoir in June 1966, it quickly became aerated in the Lehigh River, and for 40 miles downstream from the dam the water was nearly saturated with oxygen. Most of the river water requires only moderate treatment for industrial use and public distribution throughout the Lehigh River valley. At times, however, some segments of the main river and its tributaries transport industrial wastes and acid coal-mine drainage. Usually the relatively high concentrations of solutes in water and the ensuing damage caused to quality by such waste discharges are more extensive and prolonged during droughts and other periods of low streamflow. For many years the Lehigh River flow has been continuously measured and its water chemically analyzed. Since May 1966 an instrument installed by the U.S. Geological Survey at Easton, Pa., has continuously recorded such water-quality parameters as specific conductance, temperature, and dissolved oxygen content.