Recent phylogeographic studies in several organisms have elucidated the varied effects of Pleistocene glaciations and of Pre-Pleistocene events on populations from glacial refuge areas. This study investigates those effects in riparian trees (Populus spp.), whose particular features may convey enhanced resistance to climate fluctuations. We analysed the phylogeographic structure of white, black and grey poplars (44, 13 and two populations, respectively) in the Iberian Peninsula using plasmid DNA microsatellites and sequences, we assessed fine-scale spatial genetic structure and the extent of clonality in four white and one grey poplar populations using nuclear microsatellites and we determined quantitative genetic differentiation (Q_ST) for growth traits among populations and river systems in white poplar. Black poplar displayed higher regional diversity and lower differentiation than white poplar, reflecting its higher cold-tolerance. In white poplar, the Atlantic and Mediterranean drainage basins but also river basins were strongly differentiated, indicating confinement to the lower river courses during glacial periods and moderate gene exchange along coastlines. In this species, the northern Iberian river basins had lower diversity, fewer private haplotypes and larger clonal assemblies than the southern basins, indicating a stronger effect of glaciations in the north. The dependence of white poplar on phreatic water was evidenced by stronger isolation by distance among river basins than within them. Despite strong genetic structure and frequent asexual propagation in white poplar, some growth traits displayed adaptive divergence between drainage and river basins (Q_ST > F_ST), highlighting the remarkable capacity of riparian tree populations to adapt to regional environmental conditions.