Savanna regions may contain significant areas of forest, usually in riparian environments. We described forest composition and quantified environmental variables for 80 plots in the watershed of the Yucao River (2550 km(2)), a tributary to the Meta River (Orinoco basin) in the eastern plains of Colombia. The total sampled area of 3.2 ha contained 147 tree species (110 genera and 45 families) with diameter at breast height > 5 cm. The families represented by most species were Leguminosae (15 species), Arecaceae (11 species), Rubiaceae (9), Chrysobalanaceae (7), Euphorbiaceae (7), Melastomataceae (7) and Myrtaceae (7). Although most of the tree species have wide distribution ranges in humid neotropical forests, and few are endemic to the region, comparison with other neotropical forests suggests that the combination of taxa is characteristic for neotropical gallery forests. Cluster analysis allowed the definition of five forest types. They show important differences in a number of environmental variables, especially those related to inundation regimes and soil properties. Canonical correspondence analysis indicates good correlation of the first ordination axis with the size of the stream with which the forest is associated, the height to which the water rises during floods and variables related to soil fertility. The second ordination axis differentiates between soils with high or low water content in the dry season, high or low organic matter content and low or high bulk density. The results demonstrate the role of spatial variability and interactions of floodplain morphology, hydrological regime, and soil properties in structuring this important riparian community.