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combination of these factors provides greater resolution in identifying subtle genetic differences. However, these subtle genetic differences should. be interpreted carefully. Hedrick (1999) points out that when using highly variable markers like microsatellites, statistically significant differences do not necessarily reflect meaningful biological differences "both because the patterns of adaptive loci may not be correlated with highly variable loci and statistical power with these markers is so high." Historically, there may have been a low level of genetic structure within the Virgin River as a result of isolation by distance. In some systems, there is a positive correlation between geographic distance and genetic distance even within a group of organisms that are continuously distributed. This effect, referred to as isolation by distance, is due to a greater probability of matings between individuals from neighboring areas than between individuals from distant portions of the range (Wright 1943). If this low level of population structure did exist within the Virgin River, it may have been enhanced by anthropogenic changes that limited movement of fish. A fish barrier was built in 1988 near the border of Arizona and Utah to prevent upstream movement of invasive species. However, this barrier limits upstream movement of native species. In addition, there are many irrigation diversion dams along the length of the river that limit upstream passage of fish (S. Meismer, VRRMRP, personal communication). At times, the altered flow regimes caused by these diversions result in complete dewatering of portions of the river several kilometers in length (USFW S 1994) cutting off all movement between neighboring reaches. As indicated by FsT values ranging from 0.009 to 0.033 (P < 0.004), there were slight differences between the Dexter populations and the populations from 16