Fluvial planform change is the process of formation and development of new feature or reshaping of existed features by the flowing water in river system. The aim of this study was to identify, quantify and map the lower reach of Gilgel Abay River trunk 56 km from the mouth of river in to Lake Tana, planform change over three decades. GIS and recent advancement of Remote Sensing has been used to identify, quantify and map planform changes with the integration of serious interview with elder personnel and field observation. Topographic map, latest Landsat images and recent Spot images of 2016 (1.5 x 1.5) m spatial resolution were used to delineate planform change features of study reach over the last 35 years. From the channel planform change driver, anthropogenic effect is the main driving factor of serous planform change in the study reach. The intensive amount of deforestation in Gilgel Abay watershed over the study period results expansion of farm lands, bare lands and built up area where as forest, bush and shrub lands including grass lands areas decreased drastically. From the natural river process, the erosion and deposition of sediment on both banks and mouth of river which results in continuous development of delta area. The study reach undergoes different planform changes. The amount and rate of river bank migration, length, valley length, sinuosity, width, lateral migration, chars and island formation including delta area and lake shore line variation around the mouth of river into Lake Tata from 1984 to 2019 were the major planform changes identified, quantified and mapped. The main planform changes around the mouth of river were, bifurcating the channel 5 km upstream of the current mouth and development of 10.2 km2 delta areas. The study reach sinuosity was laid under low sinuous channel category with sinuosity between 1 and 1.32 and it shows general decrease of sinuosity by -4.31%. The watershed and lower reach of the river need planning and management policy to minimize river planform change including channel shifting, delta and island expansion and alteration and its effect on lake level and surrounding ecosystem imbalance.