Fission-fusion social dynamics allow animals to respond to short-term environmental changes by temporarily adjusting group size. The drivers of such complex social dynamics are thought to relate to resource availability, density effects and social interactions. During 2008-09, we collared 15 Cape buffalo (Syncerus caffer caffer) cows in different groups in the Okavango Delta, Botswana, to study social dynamics in a flood-pulsed ecosystem. Hierarchical clustering identified 2 subpopulations, 1 migratory and 1 resident. We calculated Utilization Distribution Overlap Index (UDOI) and number and duration of fusion events between buffalo dyads and related them to environmental variables. Number of fusion events and duration of fusion periods did not vary seasonally, total fusion time varied seasonally and annually, and UDOI varied with year and subpopulation. Fission events were more likely in cluttered habitats, but only in the late flood season. There was more open habitat in home range overlap (HRO) areas than home ranges (HR) in most seasons. Pan density in rainy season HROs was lower and higher than in HRs in 2008 and 2009, respectively, and in all flood seasons HRO areas were closer to permanent water than HRs, suggesting that fusion occurred when buffalo congregated on resources. Whereas previous studies described large herds that sometimes split, we identified numerous smaller groups that occasionally fused, indicating a very fluid social system. Our results highlight the need to understand social system flexibility to ensure appropriate management and understand the varying impacts of environmental and anthropogenic effects on subpopulations within the same geographic area.