Seasonal analyses of surface water geochemistry were conducted in the Clark's Fork of the Yellowstone watershed to determine whole-rock weathering rates. The Clark's Fork of the Yellowstone is a high-elevation catchment with distinct bedrock lithologies. Using dissolved solute concentrations and stream flow data, we calculated cation denudation rates of 119 g m-2 yr-1 (65,900 eq ha-1 yr-1) for carbonate-rich sedimentary rocks, 16.6 g m-2 yr-1 (8200 eq ha-1 yr-1) for andesitic volcanics, and 9.8 g m-2 yr-1 (5300 eq ha-1 yr-1) for granitic gneisses. Ca/Na ratios indicate that chemical weathering of disseminated calcite in granitic rocks contributes to the total solute load in these subcatchments. Removal of this calcite component decreased our calculated granitic weathering rate to 3.4 g m-2 yr-1 (2100 eq ha-1 yr-1). ¿ 1999 American Geophysical Union