Vegetation in the Delhi stretch of the floodplain of the river Yamuna was examined in relation to hydrological characteristics. The floodplain was delineated into four zones based on hydrological interventions. Seventy-four plant species including forty-two aquatic/semi-aquatic were identified. The decrease in water discharge from Jhangola to downstream Okhla, led to reduction in species richness. Co-structures between hydrological characteristics and vegetational composition indicated that vegetation in the four identified zones was governed essentially by hydrological factors. Zones I and III exhibited near-perfect correspondence signifying that variation in vegetational composition in these zones could be explained, to a great extent, on the basis of variations in the hydrological conditions. Zone II and IV, with relatively weaker correspondence, indicated that there were gradients other than hydrological conditions, which caused variations in vegetational characteristics. The present study highlighted the importance of allocation of water for periodic inundation to maintain floodplain characteristics including aquatic/semi-aquatic vegetation cover as critical to the management of the river ecosystem. The current policy of water use focused entirely on human uses ignoring ecological requirements, and had clear adverse implications on the health of the river ecosystem. Copyright © 2009 John Wiley & Sons, Ltd.