The effect of power plant cooling water in raising natural water temperatures at a location near the power plant on the Patuxent River estuary is clearly evident from thermograph records. Surface temperature at a station 333 m (1,000 ft) downstream from the discharge canal was raised an average of about 4 C, and at times by as much as 8 C. Temperature rises were greatest during the winter. Infrared imagery showed that elevated surface temperatures could be detected about 5.5 km (3 nautical miles) upstream at flood tide. Temperature profiles obtained from airborne radiation equipment revealed a complicated surface temperature pattern and also showed the effects of density differences and wind action on the steam-electric station (S.E.S.) effluent plume. Mean annual salinity for a 5-year period (1963–1967) was highest in 1966, about 12.3 ‰, and lowest in 1967, about 9.9‰. Dissolved oxygen values for 1966–1967 ranged from 3.2 to 15.6 mg/l, and saturation ranged from 55 to 152%. Turbidity levels were inversely related to salinity, with the highest annual, mean of 28 JCU (Jackson Candle Units) occurring in 1967, the lowest salinity year. The extreme tide range was 2.1 m (6.7 ft); mean water levels at the Patuxent Bridge were highest in summer and lowest in winter. Water stages are more affected by wind speed and direction than by flow in the river.