Periods of population influxes during winter recreation occur simultaneously with periods of extreme low flow in many Rocky Mountain areas. The ability of streams to assimilate additional nutrient loading is reduced by the low-flow conditions. Low-flow water-quality characteristics of the Slate and East Rivers, which drain the Crested Butte area, were investigated in December 1996. Six sites were chosen for evaluation-four on the Slate River and two on the East River-to assess water-quality conditions, including nutrient (nitrogen and phosphorus) concentrations and algal biomass, during a 24-hour period. Discharge in the Slate River ranged from about 18 to 30 cubic feet per second, and discharge in the East River downstream from the mouth of the Slate River was about 80 cubic feet per second. Chemical concentrations in water in the Slate and East Rivers generally were dilute with specific-conductance values of 175 to 300 microsiemens per centimeter and alkalinity values of 40 to 110 milligrams per liter during low-flow conditions.
Dissolved oxygen was at or near saturation at all measurements sites. Ammonia nitrogen concentrations increased downstream from Crested Butte and Mount Crested Butte in the Slate River and then returned to background concentrations in the East River. Concentrations of nitrite plus nitrate nitrogen increased downstream from the Crested Butte area, probably associated with the nitrification of the ammonia to nitrate, and concentrations then were diluted in the East River downstream from the confluence of the Slate River. Phosphorus concentrations also increased slightly in the reach downstream from Crested Butte and Mount Crested Butte.
Algal biomass values increased downstream from the Crested Butte area, decreased to low values in a subsequent reach, and then returned to higher values downstream. Biomass values were similar to those found in unenriched to moderately enriched streams. The lower biomass and higher phosphorus values occurred in a reach that was covered completely with ice and snow. Algal biomass in this reach was extremely low, probably due to the absence of light. The biomass values upstream and downstream from this reach were moderately high and probably resulted in the lower phosphorus and possibly somewhat lower nitrogen, which suggests that benthic algae may be partially controlling the nutrient levels through assimilation and uptake. When light conditions restrict algal growth and subsequent loading occurs, the concentrations of phosphorus increase slightly. Once the physical limitation (absence of light) is removed, the biomass responds with a corresponding decrease in phosphorus.
The nutrient concentrations were low and well below stream standards. Nutrient increases were measured downstream from Crested Butte and Mount Crested Butte, and these increases resulted in an increase of algal biomass. Overall results indicate that, at the present time, the Slate and East Rivers can assimilate winter low-flow nutrient loads.