Macroinvertebrate and algal community samples were collected during a 3-year period at sites located near Guanella Pass, Colorado, to provide baseline characterization data. Water-quality sampling and habitat evaluations were used to aid in the interpretation of the biological data. The study was part of an environmental investigation for a proposed roadway reconstruction project on Guanella Pass. Discharge was strongly affected by snowmelt during May-July. Habitat scores were optimal (147-199), as determined by U.S. Environmental Protection Agency Rapid Bioassessment Protocol methods. Generally, low median concentrations of nitrogen and phosphorus (less than 0.02 milligram per liter) were detected at all sites. The water temperatures ranged from 0.4 to 11.2 degrees Celsius. The average pH for all sites was neutral, and specific conductivities were dilute (less than 160 microsiemens per centimeter at 25 degrees Celsius). The median suspended-sediment concentration was less than 20 milligrams per liter at all sites. During the study, 100 macroinvertebrate taxa were identified. The dominant taxonomic groups of macroinvertebrates were Diptera (true flies), Ephemeroptera (mayflies), Plecoptera (stoneflies), and Trichoptera (caddisflies). Macroinvertebrate density ranged from 6.6 to 4,300 organisms per square meter at sampled sites. Shannon-Weaver diversity values for the macroinvertebrate samples ranged from 1.6 to 4.5. Collector-gatherers dominated the functional feeding groups at most sites. Average abundance of Ephemeroptera, Plecoptera, and Trichoptera (EPT) was 56.7 percent; EPT:Chironomidae ratios were greater than 2:1 for every site except during water years 1996 and 1997. Chironomids were greater than EPT at four sites in water year 1996 and at one site in water year 1997. The percentage of macroinvertebrate community similarity between site pairs varied from 0 to 80 percent. The number of algal taxa identified was 280. The dominant algal divisions, in terms of density, were Cyanophyta (blue-green algae), Chrysophyta (diatoms), Chlorophyta (green algae), Rhodophyta (red algae), and Euglenophyta (euglenoids). In general, diatom biovolumes dominated the algal assemblage, followed by blue-green algae, green algae, red algae, and euglenoids. Algal densities ranged from 3.1 X 102 to more than 4.7 X 106 cells per square centimeter, and algal biovolume ranged from 2.3 X 104 to 4.6 X 109 cells per cubic centimeter. Diversity values for diatoms ranged from 1.5 to 4.9. The pollution tolerance index (PTI) for diatoms ranged from 1.8 to 3.0. Sensitive diatoms were present at each site and ranged from 21 to 97 percent. The percentage of motile diatoms ranged from 0 to 13 percent. The presence of acid-tolerant diatoms ranged from less than 0.5 to greater than 20 percent. The percentage of community similarity between site pairs ranged from 1 to 97 percent. Overall, the biotic metrics that were evaluated during this study indicate that the macroinvertebrate and algal communities in the streams on Guanella Pass are not degraded by the existing road. Erosion may cause some localized effects but may not affect the overall health of the whole stream system. The degraded condition of Geneva Creek probably is due to natural effects as opposed to road effects. Although upper South Clear Creek, upstream from Naylor Creek, is located downstream from several sources of road runoff, the biological community at this site does not seem to be negatively affected.