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9 Valdez (1990) found that densities of red shiner, sand shiner, and fathead minnow increased with decreasing flow peak, but concluded that management of flows would only temporarily control these nonnative fishes because their high reproductive potential would enable them to recover within 2-3 years. Information from other basins substantiates how nonnative fishes are influenced by the environment For example, Cooper (1983) sampled fish populations in the Pit River in northeastern California. Introduced fishes dominated lower river reaches that had been altered by human activity. The lower reach had been altered "by impoundments and diversions and had sluggish flows and low gradient Upper undisturbed reaches contained native fish assemblages. The upper reach was characterized as unaltered with high stream velocity, steep gradient, and riffle, run, and pool habitats. The altered habitat was ecologically similar to the natural environment of the introduced species. Organisms living in habitats subject to frequent perturbations may evolve strategies or behaviors that minimize the impact of, or exploit, the natural disturbance (Meffe 1984). Fishes that evolved in Southwestern streams would be expected to have evolved mechanisms to avoid or utilize frequent flash floods. Meffe (1984) found that introduced mosquito fish incurred greater loses during floods than native Sonoran topminnow Poeciliopsis occidentalis. Native topminnows exhibited behaviors such as quick response and proper orientation to flood events that enabled them to persist ,Flood events were augmented in the last 80-90 years by arroyo cutting which deeply entrenched many streams. Where topminnows and introduced mosquitofish were sympatric, mosquitofish were usually eliminated within a few months by floods. He suggested that apparent coexistence of topminnow and mosquitofish was allowed by periodic flash flooding that removed large numbers of predatory mosquitofish. He supported this by comparing fish community structures in several flooding and non-flooding habitats. Topminnow were eliminated or severely reduced in areas not prone to flooding, but coexisted with mosquitofish in areas that frequently flooded. In arid, mountainous regions of Arizona and New Mexico, nonnative fishes that evolved in lowland, mesic conditions were intolerant to natural southwestern flooding regimes whereas native species resisted floods (Minckley and Meffe 1987). They hypothesized that different responses probably reflected different evolutionary histories. Nonnatives were displaced during floods from narrow canyon reaches to wider floodplain reaches where they persisted. A few species affected by high-intensity flooding included: common carp, red shiner, fathead minnow, mosquitofish, largemouth and smallmouth bass green sunfish, bluegill, and catfish. Red shiner and mosquitofish rapidly reestablished populations due to high reproductive potentials. SmaI1mouth bass and green sunfish, both stream adapted fish, reappeared within weeks after flooding. Regulated releases in low, controlled volumes apparently had little affect on nonnative fishes. Native fishes might not always be at an evolutionary advantage during perturbations, such as flood events. If introduced species evolved within an area that was also subject to flooding, then the introduced fish might be at an adaptive advantage over native fishes.