Base flow chemical signals were used to determine the weathering reactions that control the groundwater chemistry of a geologically heterogeneous, mountainous watershed. Major ion signatures result from cyclic salt and formation water inputs and from the weathering of easily oxidized or highly soluble minerals, such as pyrite, calcite, and dolomite. If montmorillonite is the dominant secondary mineral product, then the bulk of silicate weathering probably involves volcanic rocks. Spatial rates of base flow chemical denudation range from 0.0004 to 0.02 mm yr-1, and temporal rates range from 0.0006 to 0.6 mm yr-1. The mean chemical denudation rate for the watershed is 0.03 mm yr-1, which is comparable to some of the world's most rapidly weathering large drainage basins. Because highly soluble or easily oxidized minerals contribute the bulk of the chemical signal to basin waters, spatial and temporal rates of chemical denudation are constrained largely by recharge and discharge rather than local variations in lithology. ¿ 2001 American Geophysical Union