River embankments are one of the main causes of a loss in active channel movement. The knock-on effects of river channel stabilization include important shifts in the ecohydrological conditions in the protected zone behind embankments, which has already been well exemplified. However, few authors have attempted to investigate changes to pioneer units of vegetation colonizing the active channel following the construction of dikes. As a consequence we studied the structure, composition and diversity of pioneer vegetation units dominated by willow and poplar along the River Drôme in two adjacent, but contrasting, river reaches: an embanked reach and an unconstrained reach, each subject to the same flow regime, but offering very different hydrogeomorphological contexts for vegetation colonization. Our results clearly demonstrate that river embankments affect channel geometry. A reduction in channel width (from between 200 and 500 m to less than 100 m) leads to steeper elevational gradients with an overall coarser grain size and consequently drier overall conditions on colonizable landforms in the channelized reach. Pioneer vegetation responded by colonizing landforms at higher elevations above the low water mark and by occurring in smaller units within the channelized study reach. There was also a reduction in species diversity, a homogenization in vegetation unit structure and a predominance of more drought-tolerant ligneous species (P. nigra) in pioneer vegetation units in the channelized study reach.