Abstract

A small watershed which includes several small lakes in the Uinta Mountains of northern Utah is underlain by monolithologic Precambrian siliciclastic rock with extremely limited buffering capacity. In spite of this, systematic spatial and temporal variations in alkalinity, pH, and major elements occur in springs, small streams, and one lake within the watershed. Alkalinity changes by 3-fold over 1 km of vertical elevation within the watershed. Cations and silica show similar although less dramatic changes with respect to elevation. Silicate weathering seems to constitute the dominant mechanism of alkalinity generation. The water column in Marshall Lake shows significant seasonal stratification of temperature and chemical constituents. Vertical variation of alkalinity in the lake during the summer is related to the input of geochemically distinct water sources in the watershed. Vertical pH variations in Marshall Lake do not match alkalinity variations but instead are related to photosynthesis in the upper and middle portions of the water column. Thermal and chemical stratification is greater in the lake than in well-studied alpine lakes of the Sierra Nevada. Numerical hydrodynamic models suggest that temperature and chemical stratification of this Uinta Mountain lake can be attributed to lake depth (13 m vs. more shallow depths of the Sierra lakes) or relatively weak wind shear stress. The combined watershed/lake study demonstrates the need for complete vertical water sampling in order to accurately characterize the geochemistry of deep (> 10 m) alpine lakes.