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field conditions when water temperatures depart far from 20°C (Marsh, 1985; Kaeding and Osmundson, 1990). If estimates of climate during full glacial in the lower Colorado basin vs. the Colorado Plateau are correct (Betancourt, 1990; van Devender, 1940), such thermal problems would have existed upstream but not downstream. Self-sustaining populations of razorback suckers would have therefore been excluded from much of the upper mainstem Colorado and Green rivers and their west-flowing tributaries during this time period, while those downstream enjoyed aloes-altitude, low-latitude thermal refugium (van Devender, 1990). Due to recency of establishment (<10 Ka), upper basin populations would be expected to exhibit levels of within and among population diversity too low to produce observed downstream diversity. Consideration of climatic conditions described above also leads to an alternative hypothesis where populations of the lower basin (i.e., Lake Mohave and the Gila River) may have been a stable "source" of colonizers continually invading the upper basin following thermal amelioration (Figure 3). The observed clinal reduction in diversity could result from recent serial bottlenecks as razorbacks colonized further upstream after the last glacial advance. If the system has not achieved equilibrium, upstream populations would exhibit only a subset of variation found in their downstream source, with the most common haplotype(s) likely to become more common as one moves upstream. Given adequate time, continuing dispersal up- and downstream would homogenize populations, resulting in an equilibrium where all populations exhibit similar levels of variation and sets of haplotypes. Even if the system has achieved equilibrium, however, the observed cline in diversity could be generated by a concordant cline in population size also decreasing upstream. Given our knowledge of the biology of "big-river" fishes, such a cline might be expected. These species are seriously impacted by cold water, as indicated by their failure to reprQtiuce and 13