This study attempted to predict the likely impacts of a dam reservoir and flow regulationrnon riparian plant composition and diversity. The study was conducted around the Kogarndam in the upper part of the Blue Nile basin in the northwestern part of Ethiopia. Floristicrncomposition and diversity in the riverine and adjacent sites of the river was studied. Thernpresence of plant species that would be affected by the storage of the Koga River waterrnand the modification of the downstream river flow regime were assessed. A total of 87rnplots (3.48 ha) were laid, of which 57 (2.28 ha) were located in the upstream area, 17rn(0.68 ha) in the vicinity of the reservoir, and 13 (0.52 ha) in the downstream area. A totalrnof 71 species were identified, of which 36.7 % were trees, 26.8 % shrubs, 18.3 % werernherbs, and 4.2 % climbers. About 59 species of vascular plants, belonging to 39 familiesrnand 53 genera, were recorded upstream of the dam. 17 species comprising 15 familiesrnand 17 genera were recorded in the area that will be flooded by the reservoir and 21rnspecies comprising 15 families and 18 genera were recorded in the riparian zonerndownstream of the dam. Comparison of species indicates that though there is nornsignificant difference between the three sites in terms of species density (p = 0.345). Therndownstream region has more diverse and evenly distributed species compared to thernupstream and reservoir areas. There was no significant difference in terms of speciesrndensity and diversity in the adjacent and riverine parts of the upstream and downstreamrnareas. Most of the species in the area downstream of the dam including Dodonearnangustifolia, Ensete ventricosum, Syzygium guineense, Accacia spp., Combretumrnadengonium, and Combretum molle, which were found exclusively in the riverine part,rnare species that are normally found inhabiting riverine areas. This indicates that thernmajority of the downstream vegetation prefers moisture rich substratum. Flow historyrnwas examined to analyze the likelihood of disturbance in scenarios with and without flowrnregulation. The hydrological analysis shows that the dam may lead to reductions inrnmaximum daily discharges and flood frequency. Reductions in flood peaks and over-bankrnflows could result in changes in species composition and diversity of plants growing inrnwater or water rich substratum below the dam.