The largest rivers that drain the Argentine Andean Ranges are characterised by incised valleys in high mountains and by a variety of Quaternary terraces. Their convex-up morphology suggests that these terraces are related to a series of alluvial fans developed where the tributaries join the main river. The succession of alluvial aggradation and degradation is controlled by local base level variation conditioned by temporary lake development in the main river valley. All these factors give rise to the segmented morphology of the fan surface, yielding a telescopic relationship. The variation in the morphology and number of terraces suggests that they are not controlled by a general base level. Neither tectonic activity nor significant climatic changes account for the alluvial fans at the confluence of the tributaries and the main river. Significant variations in rainfall or thunderstorms induced by the El Niño Southern Oscillation (ENSO) could explain the genesis of these telescopic alluvial fans