Floodplains of rivers in semi-arid areas of Africa are important refuge grazing areas and biodiversity spots. However, they are sensitive to changes in flood regimes. Vegetation data were collected in transects to capture inside and outside of the upper Tana River floodplain. The mean densities of the woody species and the basal areas of the tree species were determined and compared using a t-test for the vegetation inside and outside the floodplain. The importance value (IV 200) of the tree species was determined by summing up percentage relative density and relative basal area. Three groups of species were separated; those that occur in the floodplain only, those outside the floodplain only and those both inside and outside, with Rinorea elleptica, Commiphora riparia, and Prosopis juliflora having the highest density respectively. The invader Prosopis juliflora had a significantly higher mean density (t = 5.44, P = 0.00) and lower mean basal area (t = -2.24, P = 0.03) inside the floodplain. Prosopis juliflora contributed 33.2 to IV 200 and Vachellia tortilis 19.6, the highest inside and outside the floodplain respectively. The results show that changes in flood regime as a result of mega dam projects upstream and climate change can potentially modify species dynamics and invasion.