A study of water quality and bulk atmospheric deposition in the Guanella Pass area during water year 1995 (October 1994 to September 1995) has provided information to characterize conditions in the South Clear Creek and Geneva Creek Basins about 40 miles west of Denver, Colorado, prior to a possible roadreconstruction project. Analytical results of water samples collected from streams, ground water, and lakes and reservoirs indicate that water in the study area generally contained small concentrations of dissolved solids (low-flow stream sample median, 43 milligrams per liter; ground-water median, 104 milligrams per liter) and suspended sediment (high-flow stream sample median, 115 milligrams per liter). Nutrient, trace-element, and organic-carbon concentrations also were small. Acidic pH and large trace-element concentrations were associated with areas affected by natural acid-sulfate weathering or mining. Increases in particulate nutrients, total recoverable trace elements, and suspended sediment were related to seasonal snowmelt and thunderstorms. Benthic macroinvertebrate and periphyton (algae) samples collected from selected sites in August 1995 indicated large variability in characteristics among sites. Macroinvertebrate density ranged from 6.5 to 1,300 organisms per square meter, and periphyton biovolume ranged from 0.023 to 4,700 x 10⁶ cubic microns per square centimeter. The discharge of road runoff from any single drainage feature (ditch or culvert) was small compared to streamflows. Analytical results from samples of road runoff indicated a large range in specific conductance, pH in a neutral range, large values of turbidity, and calcium bicarbonate type water. Sodium or magnesium and chloride, however, were predominant in some samples and were probably related to applications of sodium and magnesium chloride to the roads. Although dissolved nutrients and trace elements were detected in samples of road runoff, particulate forms had the largest concentrations. Suspended-sediment concentrations in road runoff were large, and the sediment was primarily fine grained. Large concentrations of dissolved and total recoverable nutrients, total recoverable trace elements, total organic carbon, and suspended sediment in road runoff could cause increased concentrations in streams, lakes, and reservoirs. Ground-water concentrations of chloride, nitrite plus nitrate, and dissolved iron and manganese·could increase if the large concentrations present in road runoff infiltrate shallow aquifers. Bulk atmospheric deposition was collected at three sites with a paved road surface, five sites with a gravel or dirt surface, and five sites with collectors at least 500 feet away from a road. The median bulk -atmospheric-deposition rate for gravel (or dirt) roads was 106 times the median rate at a reference site (located at least 500 feet away from a road) and 39 times the median rate at a typical paved road site. If a stream, lake, or reservoir is located near the road, bulk-atmospheric- deposition products may settle directly onto open water or be washed in by surface runoff. Because the predominant road-runoff trace-element phase is particulate, potential toxic effects on receiving water may be minimal when compared to aquatic-life standards and regulations in the State of Colorado, which are primarily based on dissolved concentrations. U.S. Environmental Protection Agency primary maximum contaminant levels for drinking water were exceeded for dissolved uranium (proposed) in one ground-water sample.