Lithologic and climatologic controls of river chemistry

The chemistries of rivers draining a variety of lithologic and climatic regions have been surveyed for the purpose of quantifying the fluxes of bicarbonate and silica from rivers with respect to bedrock lithology and runoff. In all, 101 different rivers, each draining a primary lithology, were examined across the United States, Puerto Rico, and Iceland. To minimize seasonal effects, only rivers with at least two years 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 among rivers draining basaltic catchments shows that the river compositions define slopes which are consistent with an equilibrium (ion exchange) control on cation ratios. Among different lithologies, unique weathering rate relationships were developed with yields at typical present-day runoff rates (1-100 cm/y) increasing in the order sandstones, granites, basalts, shales, and carbonates. The bicarbonate and silica fluxes for each of these lithologies have been quantified for use in global studies of chemical denudation. Our study confirms that the dissolved yield of a given drainage basin is determined by a balance between physical and chemical weathering; thus, a warm, wet climate, or the presence of abundant vegetation cannot guarantee high rates of chemical denudation unless accompanied by high rates of physical removal.