Water qualities of two natural streams (Buffalo and Kings Rivers), one new coId-tailwater (Beaver), and two old coId-tailwaters (Norfork and Bull Shoals) in northwestern Arkansas were studied from July 1965 through October 1968. The essential difference between the old cold-tailwaters and natural streams is a change in water quality which allows the development of a new productive ecological environment. Features which typify the old tailwaters are as follows: (1) relatively homioithermal temperatures; (2) stream beds scoured by strong hydoelectric power generation currents; (3) abundant phytoplankton and benthic macroinvertebrates; and (4) absence of warm water game fishes. Environmental factors characterizing natural streams are as follows: (1) high summer temperatures; (2) seasonal and individual current fluctuations at the various stations; (3) a greater variety of benthic macroinvertebrates and ichthyofauna; (4) abundant zooplankters; and (5) a tendency toward an equal distribution of the phyla Chrysophyta, Cyanophyta, and Chlorophyta. By October 1968, the new Beaver coId-tailwater had lost all of its warm-water characteristics but had not developed the biotic features of the old tailwaters
