Tropical rivers have important role in sediment and nutrient transfer to oceans and coastal areas, sediment storage in continental basins and in the global hydrological cycle. The information on sediment concentration in rivers is important for design of reservoirs and floodplain environmental applications. Ribb and Gumara rivers are among tropical rivers which yields high sediment concentration to Lake Tana in the Upper Blue Nile (Abay) basin, Ethiopia. To determine the aggradation/ sedimentation rate of both Gumara and Ribb river floodplains dendrochronology method is used. To determine the spatial variation of sedimentation on Gumara floodplain, sedimentation pads were installed in four locations perpendicular to the river. Historical images of Google earth for the years 1991, 2010, and 2020 were used to identify the paleochannel of rivers. By using the area of floodplain and rate of floodplain sedimentation the impact of floodplains on the sediment budget in to Lake Tana was determined. The result shows that Gumara and Ribb floodplains have average sedimentation rate of 3.44cm and 4.59cm per year respectively; Whereas, the amount of sediment deposited on Gumara and Ribb floodplains is 5.85 and 20.85 million cubic meters of sediment per year respectively. The spatial variation of sedimentation has a decreasing trend as it goes far from the river banks. The land use/ land cover of watershed, amount of flow in the rivers, circularity index and human activities are the factors which can alter the rate of sedimentation. Continuous migration of river path causes the development of floodplain by point bar accretion and vertical accretion. The existence much floodplain sedimentation is no doubt due to the presence of high erosion at the upstream and high overbank flow at the downstream of the catchments. So that erosion protection measure and flood protection measures are required at upstream and downstream of the catchments respectively. Also, further study is necessary in order to obtain more knowledge in the catchments.