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MAJOR FINDINGS Urban Development is a Water-Quality Issue Within the Southern Rocky Mountains Recreation in the mountains of Colorado is becoming a significant land-use activity in many areas. Urban development and the infra- structure to support recreation have increased substantially in the last decade. Between 1990 and 1997, the population increase in Summit and Eagle Counties exceeded 40 percent (http://www.colorado. edu/l i brariesl govpu bsl co 1 on um bl counties.htm). In addition to resi- dent population increases, sea- sonal population fluxes due to recreation are large. These tempo- rary population increases occur in winter during periods of extreme low flow and minimal dilution in rivers and streams. Although urban development may be only a small percentage of total watershed area, development and transportation systems tend to be adjacent to riparian areas in mountainous terrain (fig. 6). This land-use pattern has placed human populations in locations that have the greatest effects on the quality of the water resources. Surface- and ground-water studies were imple- mented to investigate the water- quality issues in urban areas of the Southern Rocky Mountains. Surface-water studies investi- gated the effects of urban land use. Two stream-monitoring sites were located in areas of urban development (fig. 6). The Gore Creek site is downstream from Vail, and the East River site is downstream from Crested Butte. Development in Vail is approach- ing build-out, whereas there is potential for large increases of development in the Crested Butte area. The Colorado River below Baker Gulch, a reference site with little urban development in the Southern Rocky Mountains, was also sampled monthly to provide a comparison with other sites. Nutrient concentrations at sites in areas of urban develop- ment were greater than concen- trations at the reference site. Concentrations of nitrite plus nitrate and total phosphorus were slightly elevated at Gore Creek and East River compared to reference concentrations (fig. 7). Concentra- tions for nitrate were below the 10-mg/L Colorado instream stan- dard at all three sites. Un-ionized ammonia concentrations, com- puted from dissolved ammonia, pH, and temperature, did not exceed State instream standards. Dissolved and orthophosphate phosphorus concentrations were greater at Gore Creek than the other sites. Gore Creek and East River are not wastewater-eftluent- dominated streams such as might be found in large metropolitan areas; however, small amounts of nitrogen and phosphorus can increase algal growth and eutrophi- cation processes. The amount of algae and the types of aquatic invertebrates (insects) are influenced by nutri- ent enrichment from urban sources. The amount of algae (algal biovolume) determined from algae samples collected in 1996 and 1997 was largest in Gore Creek (fig. 8), where nutrient levels were higher. The percentage of the inver- tebrate community represented by pollution sensitive insects Southern Rocky Mountains Physiographic Province Colorado Plateau Physiographic Province o 20 I o 20 40 . Streambed-sediment sampling site and number In non-mining area fl::,. Multiple streambed- sediment samplIng sites and numbers In mining area Stream mOnltonng site Agnculture land use Figure 6. Sampling sites and study areas were selected to assess effects of urban mining, and agricultural land uses on water quality. ' 6 Water Quality in the Upper Colorado River Basin