This investigation was made to figure out the interaction between the surface water andrngroundwater bodies in Lake Tana sub-basin using isotope and geochemical approach.rnThe main objectives of this investigation were:rnÃƒÂ€Â‚Â™ To map the spatial and vertical distribution of water geochemical composition in the area.rnÃ°Â€Â‚Â™ To identify the origin & evolution of the isotopic and geochemical characteristics of thernwater in the Lake Tana sub-basin.rnÃ°Â€Â‚Â™ To investigate the interaction between surface water and groundwater.rnIsotope and Geochemical method has indicated that two major evolution processes of the salinityrnoperate in Lake Tana sub basin. These are evapotranspiration on the the flat plain and Deeprncirculation associated with silicic dykes.rnEvapotranspiration is the main cause of Pockets of highly saline areas in the permanent andrnseasonal parts of the wetlands. It causes water to be transported by a plant through capillaryrnaction of vessels from the soil to the air by leaving dissolved matter with no net isotopernfractionation causing salinity.It is mostly observed at the Dembia and Fogera plain close to thernwetlands and trees. It is not distributed uniformly but random and is moderately depletedrnisotopically.rnThe saline water associated with deep circulation is found close to silicic dykes. Fluid movementrnunder pressure change due to the occurrence of the dyke and osmotic processes has oftenrnresidual components because of the long relaxation time of the pressure gradient and thus salinityrnoccurs.rnThe shallow pocket salinities observed in this study may be exacerbated if other additionalrnprocesses contribute to salinity of the area. Therefore, a comprehensive groundwaterrninvestigation is a priority to implementing large schemes such as irrigated agriculturalrndevelopment in the flat wetlands. This can be done by implementing a micro geo-morphologicalrninvestigation in combination with diurnal water quality monitoring.