<p>In this study the potential alkali-silica reactivity of fluvial aggregates sourced from various lithological units within a river basin in northwestern Argentina was investigated. Natural coarse aggregates from quarries located in different sectors of the basin were analyzed. Petrographic characterizations, estimations of potentially reactive components, and expansion tests (accelerated mortar bar tests, AMBT; conventional concrete prism tests, CPT; and accelerated concrete prism test, ACPT) were conducted. Petrographic analysis revealed a predominance of metamorphic rocks (mainly schists and phyllites), with significant amounts of microcrystalline and highly strained quartz, identified as the primary reactive components. Minor amounts of opal, chert, and chalcedony were also detected, though their influence on expansion was limited. Basaltic rocks, present in high proportions in some quarries, appear to reduce the overall aggregate reactivity by diluting the reactive metamorphic components. AMBT indicated high reactivity in six of the seven quarries. However, CPT and ACPT qualified only three quarries as moderately reactive. The results reveal a clear relationship between the lithology of the source areas and the reactivity of the aggregates. High reactivity was observed in aggregates derived from areas with a greater proportion of metamorphic rocks, characterized by a high content of microcrystalline and highly strained quartz. In contrast, the lowest expansion values were associated with quarries dominated by basaltic rocks.</p>