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The floodplain ecosystem of the Nyl River is located in a semi-arid region of South Africa where water limits both human development and ecosystem functioning. Proposed upstream impoundments threaten the ecosystem’s functioning and hence its conservation value and eco-tourist potential. Articulation of these threats to the floodplain ecosystem requires a predictive understanding of the relationships between the biota and hydro-geomorphic processes. This study provides the basis for the establishment of these relationships by presenting a description of the plant assemblages of the Nyl River floodplain, identifying environmental correlates of this vegetation pattern, and by distinguishing functional plant groups. A correspondence analysis identified three major vegetation associations on the floodplain ecosystem: near-channel sites, hydromorphic sites and sodic sites, Geomorphic landform type, soil moisture and soil texture were the best correlates of the vegetation pattern. Elevation above the channel and distance from the channel were poor correlates of vegetation pattern. The distribution of functional groups, which were defined by plant life forms, showed that the near-channel sites were dominated by prostrate and decumbent grasses, hydromorphic sites by erect grasses, and sodic sites by a variety of succulent life forms. The plant-environment relationships recognized suggest that future distributions of functional plant groups may provide a sensitive index of the impacts of anticipated reductions in run-off.