From preface: This monograph presents a detailed description of the results of an ecological study, 1965-1973, of the effects of the warm-water effluent from the Connecticut Yankee Atomic Power Company's electric generating plant at Haddam Neck on the lower reaches of the Connecticut River, USA; it is accompanied in the Introduction by summary recitals of associated research projects and monitoring programs - microbiology, impingement of fish on the intake screens of the plant, and environmental radiation. The study, which embodies hydrography, the benthos, and the resident and anadromous fishes, is characterized by the fact that it was begun while the plant was still in the early stages of construction and that the field work was only terminated nearly six years after the inception of commercial operation. Thus there were essentially three years for the collection of base-line data on the biota under 'pre-thermal' conditions for comparison with the situation as it was altered by the large flow of condenser-cooling water from the generating station.