Abstract
As a result of ongoing human induced or natural factors acting on river channels, banks, and within a catchment, alluvial river systems change their course and morphology over time. The base level changes and the backwater effects affect rivers entering a static water body. For coastal rivers, the planform changes are highly pronounced at the fluvial deltas and flood plains. Aggradation, degradation, progradation, meandering, and formation of islands and distributary channels are common processes on coastal rivers. This study investigates planform changes and landscape responses of Gilgel Abay river about 36 km stretch starting from the bridge, near Chimba, to its entrance to Lake Tana by using historical images for the last 60 years (1957-2020) and field observation. The study reach was divided in to three parts based on feature characteristics. For data preparation and analysis, image analysis software's; ENVI, ERDAS IMAGINE, and ArcGIS were used. From land use land cover classification, the land use pattern near the river flood plain and delta area changed significantly. The overall planform results (sinuosity, width, and island) show that, Gilgel Abay River within the study reach has shown little change in its planform for the last 60 years. However, the landscape of alluvial delta formed at the mouth of the river has shown huge change. Accretion-erosion result map shows, a maximum of 18.73 ± 1.97 m/y accretion and -12.48 ± 1.44 m/y erosion at the right side (eastward) and a maximum of 50.06 ± 1.44 m/y accretion and lost only 3.95 ± 1.97 m/y westward. The delta area analysis shows in the last five decades, 17.13 ha/yr of land has formed disproportionally out of which over 56% is added at the right shore of the river. In general, for the planform changes that occurred in Gilgel Abay river channel and fluvial delta, human-induced factors have a great contribution. Increasing interest for new settlements inside delta flood plain, productive agricultural land and artificial lake level rise changes the river planform and the delta appearance. Quantitative and qualitative mapping of river and delta coupling with feeding basins and floodplains is important to enhance and understand the socioeconomic influences on river morphology and needs an integrated management framework.