Analysis of planform changes of the lower reach of Gilgel Abay River Upper Blue Nile Basin, Ethiopia, using remote sensing and GIS

Abstract

Fluvial planform change is the process of formation and development of new features or reshaping of existed features by the flowing water in the river system. This study aimed to identify, quantify, and map the lower reach of Gilgel Abay River reach 56 km from the mouth of the river into Lake Tana, planform change over three decades. GIS and recent advanced remote sensing have been used to identify, quantify, and map planform changes with the integration of serious field observation. Topographic map, latest Landsat images, and recent Spot images of 2016 (1.5 × 1.5 m spatial resolution) were used to delineate planform change features of study reach over the last 35 years. From the natural river process, the erosion and deposition of sediment on both banks of a river result in the continuous expansion of river width and area. The study reach undergoes different planform changes. The amount and rate of river bank migration, length, valley length, sinuosity, width, lateral migration, and lake shoreline variation around the mouth of the river into Lake Tata from 1984 to 2019 were the major planform changes identified, quantified, and mapped. The main planform change around the mouth of the river was bifurcating the channel 5 km upstream of the current mouth of the river. The study reach sinuosity was laid under low sinuous channel category with sinuosity between 1 and 1.32 and it shows the general decrease of sinuosity by −4.31%. The lower reach of the river needs 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.