Dilution, chemical precipitation, biological uptake and inflow inputs are important processes regulating stream water chemistry but quantitative evaluations of their relative contributions are rare. We examined stream water chemistry and discharge in two streams, one heavily affected by volcanic activities, one much less so, and their confluence to evaluate the importance of these processes, with special emphasis on dilution and precipitation, on reducing solute concentrations in northern Taiwan. We found that differences in the relative importance of the four processes led to three scenarios of upstream-downstream patterns of solute concentrations and fluxes. In Scenario I, solute concentration was greater but flux was smaller in the upstream than the downstream, likely due to dilution through the input of lateral inflows with low solute concentrations. In Scenario II, both solute concentration and flux were greater in the downstream, likely because the inflow water contained high solute concentrations. In Scenario III, both solute concentration and flux were greater in the upstream, likely because chemical precipitation dominated the upstream-downstream pattern. Dilution accounts for 63.7 ± 1.1% of the reduction of concentrations of all solutes in the confluence due to the input of relatively “clean” water from the less contaminated stream. Net precipitation effect was solute-specific and was positive only for Fe and PO4 3−. It reduced concentration of Fe by 79 ± 0.07% and PO4 3− by 49 ± 0.25%, likely due to decreases of solubility with increases in pH. Frequent mild rainfall is more effective than more extreme rainfall on lowering solute concentrations because the dilution effects of even large rainfall are short lived. The projected increases in drought events will reduce dilution effects when the need for diverting water from “clean” streams for anthropogenic uses is greater, making water resource management more challenging.
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