Effect of basin physical characteristics on solute fluxes in nine alpine/subalpine basins, Colorado, USA

Alpine/subalpine basins may exhibit substantial variability in solute fluxe s despite many apparent similarities in basin characteristics. An evaluatio n of controls on spatial patterns in solute fluxes may allow development of predictive tools for assessing basin sensitivity to outside perturbations such as climate change or deposition of atmospheric pollutants. Relationshi ps between basin physical characteristics, determined from geographical inf ormation system (GIS) tools, and solute fluxes and mineral weathering rates were explored for nine alpine/subalpine basins in Rocky Mountain National Park, Colorado, using correlation analyses for 1993 arid 1994 data. Stream- water nitrate fluxes were correlated positively with basin characteristics associated with the talus environment; i.e., the fractional amounts of stee p slopes (greater than or equal to 30 degrees), unvegetated terrain and you ng debris (primarily Holocene till) in the basins, and were correlated nega tively with fractional amounts of subalpine meadow terrain. Correlations wi th nitrate indicate the importance of the talus environment in promoting ni trate flux and the mitigating effect of areas with established vegetation, such as subalpine meadows. Total mineral weathering rates for the basins ra nged from about 300 to 600 mot ha(-1) year Oligoclase weathering accounted for 30 to 73% of the total mineral weathering flux, and was positively corr elated with the amount of old debris (primarily Pleistocene glacial till) i n the basins. Although calcite is found in trace amounts in bedrock, calcit e weathering accounted for tip to 44% of the total mineral weathering flux. Calcite was strongly correlated with steep slope, unvegetated terrain, and young debris-probably because physical weathering in steep-gradient areas exposes fresh mineral surfaces that contain calcite for chemical weathering . Oligoclase and calcite weathering are the dominant sources of alkalinity in the basins. However, atmospherically deposited acids consume much of the alkalinity generated by weathering of calcite and other minerals in the ta lus environment. Published in (C) 2001 by John Wiley & Sons, Ltd.