LITHOLOGIC AND CLIMATOLOGIC CONTROLS OF RIVER CHEMISTRY

The chemistries of rivers draining a variety of lithologic and climati c regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock litholo gy and runoff. In all, 101 different rivers, each draining a primary l ithology, were examined across the United States, Puerto Rico, and Ice land. To minimize seasonal effects, only rivers with at least two year s of data were used. Basaltic catchments were examined in the greatest detail. In a survey of Hawaiian Island watersheds, the average river chemistries could be related to the distribution of soil associations within each catchment. An analysis of cation activity relationships am ong rivers draining basaltic catchments shows that the river compositi ons define slopes which are consistent with an equilibrium (ion exchan ge) control on cation ratios. Among different lithologies, unique weat hering rate relationships were developed with yields at typical presen t-day runoff rates (1-100 cm/y) increasing in the order sandstones, gr anites, basalts, shales, and carbonates. The bicarbonate and silica fl uxes for each of these lithologies have been quantified for use in glo bal studies of chemical denudation. Our study confirms that the dissol ved yield of a given drainage basin is determined by a balance between physical and chemical weathering; thus, a warm, wet climate, or the p resence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal.