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2c in Figure 1). . Discriminant analysis revealed that these clusters differed from one another primarily in elevation, the amount of in-stream vegetation, substrate composition, and total suspended solids. Mean elevations for clusters 2a, 2b, and 2c, ignoring the two high elevation outlier sites', were 6522 feet, 6405 feet, and 6491 feet, respectively. Utility of Aquatic Ecoregions as Opposed to Hydrologic Units A series of ANOVAs on the 22 chemical and physical environmental variables indicated that Aquatic &" Won 1 and Aquatic Ecoregion 2 differed most significantly (P<0.001) in alkalinity, bicarbonate, growing season length hardness, elevation, total dissolved residue, and in the square root of watershed area. Statistical differences between aquatic ecoregions were also obtained (P<0.01) for magnesium, sodium, and sulfate. With respect to Aquatic Ecoregion 1, alkalinity, bicarbonate, calcium, and total dissolved residues were doubled in Aquatic Ecoregion 2; hardness, magnesium, and sodium were tripled, and sulfates were 10 times higher- in Aquatic Ecoregion 2. In contrast to the aquatic ecoregion classification, when environmental variables were compared with sites grouped into Hydrologic Units, as opposed to aquatic ecoregions, only the square root of watershed area was clearly significant statistically (P=0.0045). Marginally significant statistical differences (P about 0.05) were observed for growing season length, nitrate/nitrite nitrogen, and potassium. Thus, aquatic ecoregions defined for the upper Rio Grande drainage appeared to be superior to Hydrologic Units in stratifying the landscape into regions which differed in more of the chemical and physical attributes. Chironomid Faunal Regions Data for c mummids indicated there were 20 sites represented and 172 species (Table 2) were present. Eighteen of the sites with chironomid data also had fish data available and 17 of the sites also had environmental data available. Similarity among sites was assessed using cluster analysis on Presence/absence data for the 18 sites that also had fish data. The cluster tree (Figure 8) indicated a general increase in chironomid species diversity from sites in the upper part of the tree (lowest diversity) to sites in the lower part (highest diversity). This is probably indicative, at least to some extent, of environmental degradation at the least diverse sites. Four clusters and one outlier site were identified from the cluster tree (numbers 1 through 5 on cluster tree, Figure 8). The first group of sites ("1" on Figure 8) may be moderately to highly impacted sites, or alternatively, they may have been sampled inadequately; they had low species diversity and the mean number of chironomid species per site was 12. These sites also had a high proportion of widespread species and few unique species (found at no other sites); the mean comber of unique species per site was one. s Casias Creek in cluster 2a and the Chamita River in cluster 2b as I1