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I studied three sites in an undisturbed, third-order blackwater stream in the central Savannah River drainage of South Carolina for 2 years before and 3 years after the accidental introduction of bluegills Lepomis macrochirus. Although large numbers invaded the stream, the species declined over the next 3 years and will likely disappear soon. The bluegills had no measurable effect on the numbers of species or individuals of the stream’s indigenous fish fauna. Nearby in the same drainage, bluegills successfully colonized an impounded cooling reservoir and a downstream swamp, but they did not become established in the high-flow stream below the reservoir. Unsuccessful colonization of the two flowing-water systems indicates the importance of impoundments to successful colonization by many nonnative fishes. These data, together with examples from arid-zone streams of the American Southwest, illustrate that unimpeded flow in lotic habitats may protect against successful invasion by some introduced fishes, whereas stabilized flows may increase their likelihood of establishment.