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281 DISCUSSION Aquatic systems in isolated areas may be dominated by one type of external variable such as glaciation, but multiple external variables usually determine stream conditions. The status of fluvial trout populations is determined mainly by the internal quality of the aquatic environment. In this study, the physical variables--stream elevations, widths, depths, pool ratings, channel gradients, and streamside cover--substantially influenced fish population densities and species composition. Elevation could be less a factor than it appeared to be superficially because decreases in elevation were so closely associated with increasing stream width and depth. The study data infer that increasing water space, which is associated with decreasing channel gradients and increasing water temperatures, had more influence on increasing fish populations than did other factors. Fish species preferred certain habitat types, but some species occupied certain types of environments because they were limited in competitive ability. This is exemplified by cutthroat trout and Dolly Varden, which occupied sec- tions of streams that did not have high populations of chinook salmon and rainbow trout. Total fish populations were highest near the grass-brush habi- tat types because the two dominant fish species centered on these environments. Chinook salmon dominated the grass type areas and the rainbow trout dominated the brush type areas. Cutthroat trout numbers, however, were at their highest in channels with dominant tree cover on the banks. Some species were adapted only to certain ranges of stream gradient, and certain species usually peaked in population density at different channel gradients. In my work, erroneous interpretations of the effect of one variable on fish populations could have involved gradients, elevations, fine sediment, and pool-riffle ratios. For example, if I had considered fine sediments alone as a dominant controlling variable, the conclusion would be that increasing amounts of fine sediment in the channel would cause increased fish populations. In analyzing the combination of variables, however, as fine sediments increased in the stream segments, stream widths and depths, pool quality ratings and percent pool evaluations increased and channel gradients decreased. All of these conditions, except possibly fine sediment increases, tend to cause an increase in fish populations, thus masking the true effect of fine sediment. In analyzing any aquatic environment, all environmental variables and their interactions must be considered. In evaluating the inventory procedure as a predictor of fishery conditions, flaws became apparent. The multivariate analysis demonstrated that while