The Lake Tana Sub-basin, located in Abay Basin, Ethiopia, where flooding, upland soil and river-bank erosion, sedimentation, and contamination of water from agricultural chemicals is the target area of this study. The aim of the study was: basin morphometric characterization, development of a hydrologic (HEC-HMS) and hydraulic (HEC-RAS) models, determination of the extent and depth of flood and analysis of the effects of climate change on the streamflow in the sub-basin. Assessment and analysis of the sub basin morphometry was done using GIS and remote sensing. HEC-HMS was used to simulate the watershed response to rainfall and to predict the impact of climate change on streamflow, while hydraulic modeling was performed using HEC-RAS. The performance of the model was tested by mean relative error, NSE, and coefficient of determination (R2 ) statistical performance measures. The model is capable of capturing peak flows and hydrograph recession curves with a reasonable accuracy of mean relative error in peak flow values of -1.91 (Gilgel Abay), -1.49 (Ribb) and -9.31 (Megech). The average NSE and R2 values are above 0.75 and 0.8 for all watersheds respectively. The drainage network analysis of the sub-basin revealed that the basin is a seventh order with a dendritic drainage pattern and very fine drainage texture. The analysis of the potential effects of climate change on the long term monthly projected temperature (maximum and minimum) showed an increasing trend in the sub-basin but no trend will be expected for precipitation. The average monthly projected maximum temperature may increase from 1.42 ° C in 2050s to 2.40 ° C in 2070s for RCP4.5 scenario and the average monthly projected minimum temperature may increase from 1.96 ° C in 2050s to 3.79 ° C in 2070s for RCP8.5 scenario. Compared to the base period, the annual outflow in the RCP4.5 scenario also showed no significant trend (increase in the near future by 6.78% where the rainfall is also increased by 3.97% and decrease by 1.78% where the rainfall is decreased by 6.22% in 2070s), while an increase in rainfall by 10.57% in the near future and a decrease by 0.81% in far future resulted in an increase in streamflow of 8.92% and 13.6% respectively for the whole time horizons in the RCP8.5 scenario. Despite limitations and uncertainties associated with obtaining observations and measured parameters, HEC-HMS model can be used to simulate hydrologic processes and predict vii the impact of climate change on streamflow whereas HEC-RAS can be used for hydraulic modelling in Lake Tana Sub-basin and similar basins. Furthermore, the drainage morphometric results indicate the watershed characteristics and serve as a basis for improved planning, management, and decision making to ensure sustainable use of basin resources. Key Words: Abay basin, Climate change, Flooding, HEC-HMS, HEC-RAS, Lake Tana Sub-basin, Morphometry