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[1] Source waters and flow paths of streamflow draining high-elevation catchments of the Colorado Rocky Mountains were determined using isotopic and geochemical tracers during the 1996 snowmelt runoff season at two subcatchments of the Green Lakes Valley, Colorado Front Range. A two-component hydrograph separation using d18 O indicates that new water dominated (82 ± 6%) streamflow at the 8-ha Martinelli catchment and old water dominated (64 ± 2%) at the 225-ha Green Lake 4 (GL4) catchment. Snowmelt became isotopically enriched as the melt season progressed, complicating the interpretation of source water models. Thus old water may be underestimated if the temporal variation in d 18O of snowmelt is ignored or extrapolated from point measurements to the catchment. Two-component hydrograph separations for unreacted and reacted waters using a single geochemical tracer were not always meaningful. Three-component hydrograph separations using end-member mixing analysis indicated that subsurface flow contributed more than two thirds to the streamflow at both catchments. Talus fields contributed more than 40% of the total discharge during summer at the GL4 catchment. A conceptual model was established for flow generation based on these results. It is suggested that surface water and groundwater interactions are much more important to the quantity and quality of surface water in high-elevation catchments than previously thought.

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We characterize the seasonal variation in the geochemical and isotopic content of the outflow of the Green Lake 5 rock glacier (RG5), located in the Green Lakes Valley of the Colorado Front Range, USA. Between June and August, the geochemical content of rock glacier outflow does not appear to differ substantially from that of other surface waters in the Green Lakes Valley. Thus, for this alpine ecosystem at this time of year there does not appear to be large differences in water quality among rock glacier outflow, glacier and blockslope discharge, and discharge from small alpine catchments. However, in September concentrations of Mg 2þ in the outflow of the rock glacier increased to more than 900 meq L À1 compared to values of less than 40 meq L À1 at all the other sites, concentrations of Ca 2þ were greater than 4,000 meq L À1 compared to maximum values of less than 200 meq L À1 at all other sites, and concentrations of SO 2À 4 reached 7,000 meq L À1 , compared to maximum concentrations below 120 meq L À1 at the other sites. Inverse geochemical modelling suggests that dissolution of pyrite, epidote, chlorite and minor calcite as well as the precipitation of silica and goethite best explain these elevated concentrations of solutes in the outflow of the rock glacier. Three component hydrograph separation using end-member mixing analysis shows that melted snow comprised an average of 30% of RG5 outflow, soil water 32%, and base flow 38%. Snow was the dominant source water in June, soil water was the dominant water source in July, and base flow was the dominant source in September. Enrichment of 18 O from À10% in the outflow of the rock glacier compared to À20% in snow and enrichment of deuterium excess from þ17.5% in rock glacier outflow compared to þ11% in snow, suggests that melt of internal ice that had undergone multiple melt/freeze episodes was the dominant source of base flow.

Nitrogen saturation is occurring throughout high-elevation catchments of the Colorado Front Range. Annual inorganic N loading in wet deposition to the Front Range of ∼4 kg ha-1 yr-1 is about twice that of the Pacific States and similar to many sites in the northeastern United States. In the last ten years at Niwot Ridge/Green Lakes Valley and Glacier Lakes, annual minimum concentrations of NO3 - in surface waters during the growing season have increased from below detection limits to ∼10 μequiv L-1, indicating that these two catchments are at the threshold of N saturation. The Loch Vale watershed is N saturated, with annual minimum concentrations of NO3 - in surface waters generally above 10 μequiv L-1; annual volume-weighted mean (VWM) concentrations of 16 μequiv L-1 in surface waters are greater than that of ∼11 μequiv L-1 NO3 - in wet deposition. At these high-elevation catchments, there has been a shift in ecosystem dynamics from an N-limited system to an N-saturated system as a result of anthropogenically fixed N in wetfall and dryfall. Results from the Western Lakes Survey component of the National Surface Water Survey show that N saturation is a regional problem in the Colorado Front Range, with many lakes having (NO3 -) concentrations greater than 10 μequiv L-1. Foliar N:P ratios in bristlecone pine increase with elevation in the Colorado Front Range, indicating that at higher elevations P is translocated from foliar tissue more efficiently than N and that increasing atmospheric deposition of N with elevation is causing a change from N limitation to P limitation in the highest-elevation bristlecone pines. Current concepts of critical loads need to be reconsidered since only modest atmospheric loadings of N are sufficient to induce N leaching to surface waters in high-elevation catchments of the western United States.

It has been suggested that the major advantage of trichromatic over dichromatic colour vision in primates is enhanced detection of red/yellow food items such as fruit against the dappled foliage of the forest. This hypothesis was tested by comparing the foraging ability of dichromatic and trichromatic Geo¡roy's marmosets (Callithrix geo¡royi) for orange-and green-coloured cereal balls (Kix 1 ) in a naturalized captive setting. Trichromatic marmosets found a signi¢cantly greater number of orange, but not green, Kix 1 than dichromatic marmosets when the food items were scattered on the £oor of the cage (at a potential detection distance of up to 6 m from the animals). Under these conditions, trichromats but not dichromats found signi¢cantly more orange than green Kix 1 , an e¡ect that was also evident when separately examining the data from the end of the trials, when the least conspicuous Kix 1 were left. In contrast, no signi¢cant di¡er-ences among trichromats and dichromats were seen when the Kix 1 were placed in trays among green wood shavings (detection distance 5 0.5 m). These results support an advantage for trichromats in detecting orange-coloured food items against foliage, and also suggest that this advantage may be less important at shorter distances. If such a foraging advantage for trichromats is present in the wild it might be su¤cient to maintain the colour vision polymorphism seen in the majority of NewWorld monkeys.

A continuous climate record since 195 1 at Niwot Ridge in the Colorado Front Range shows a decline in mean annual temperature, an increase in annual precipitation amount, and a decrease in mean daily solar radiation for the summer months. The increase in precipitation amount explains about half of the 200% increase in annual wet deposition of NO,-to Niwot Ridge over the last decade. Differences in climate parameters between 1994 and 1995 (increased snow depth and decreased net energy flux to the snowpack) resulted in a 4-5-fold increase in the magnitude of solute release from the snowpack in the form of an ionic pulse. In turn, the high chemical loading of strong acid anions in the seasonal snowpack and release of these solutes from the seasonal snowpack in the form of an ionic pulse is causing episodic acidification (ANC < 0 peq liter-l) in headwater catchments at present deposition levels. Small changes in climate parameters may cause large changes in the hydrochemistry of alpine streams. The changes in climate at Niwot Ridge are not in synchrony with lowland warming in the Great Plains to the east and serve as a reminder that climate in alpine areas is driven by local conditions and may be asynchronous with regional and global climate trends.Many alpine regions are susceptible to environmental damage that will affect both their ecological health and the regional economies. Small changes in the flux of energy, chemicals, and water to high-elevation ecosystems may result in large changes in the climate, ecosystem dynamics, and water quality of these catchments (e.g. Baron 1992). For example, field and laboratory experiments have demonstrated that initial stages of snowmelt often have ionic concentrations many times higher than averages for the whole snowpack-an ionic pulse (e.g. Johannessen and Henriksen 1978; Colbeck 198 1). The magnitude of the ionic pulse may be increased or decreased by small changes in energy flux (Williams and Melack 199 1 b). In turn, the release of solutes from the seasonal snowpack may have a direct and large effect on the solute content of stream waters .