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The Idrijca is a river flowing through the Idrija Hills and Cerkno Hills in Slovenia. It is long. It rises near Vojsko, flows towards northeast and after passing through Idrija turns to the northwest. After passing through Spodnja Idrija and Cerkno it joins the Soča in Most na Soči. It has a pluvio-nival regime and belongs to the Adriatic Sea Basin.
Beneath its course the Idrijca traverses a complex bedrock sequence. The upper reaches cut through solid and bedded Triassic limestone and dolomite; the central valley is underlain by Permian greywacke, conglomerate, and ; and the lower reaches pass over Triassic dolomite interbedded with and Cretaceous limestone breccia. This geology governs the river's chemistry: waters are of the HCO₃⁻–Ca²⁺–Mg²⁺ type, with Ca²⁺/Mg²⁺ molar ratios indicating dominant dolomite weathering, and total alkalinity typically 3.9–5.1 molarity. Field pH values range from 7.7 to 8.8, and saturation indices show persistent supersaturation with respect to calcite and dolomite, so the Idrijca both dissolves and reprecipitates carbonate minerals along its course.
Isotope analysis and mass balance studies demonstrate that carbonate solvation is the principal source of dissolved inorganic carbon (DIC) in the river, supplemented by inputs of soil-derived carbon dioxide and organic-matter decomposition in spring, and offset by CO2 outgassing to the atmosphere. Partial pressures of CO2 in the water may rise to 13 times atmospheric, making the Idrijca a net CO2 emitter. Together, these hydrogeochemical characteristics highlight the tight coupling between the basin's geology, climate and biogeochemistry processes in shaping both the flow regime and water chemistry of the Idrijca.
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