Multidecadal changes in planform morphology of the confluence of River Niger and River Benue, West Africa

Analysis of 35 years (1987-2022) of time-lapse data derived from Landsat imagery and SRTM DEM has revealed significant changes in planform morphology at the confluence of the River Niger and River Benue in Lokoja, West Africa. In the study area, the Niger is flanked by high elevation plateaus of ca. 400 m altitude and low-lying floodplains on its western bank. The Benue is characterized by more abundant intra-channel and bankattached bars relative to the Niger. Over the 35-year period, a net downstream migration of 880 m was observed at the junction, its angle decreased from ca. 175 degrees in 1987 to 50 degrees in 2006 and to ca. 16 degrees in 2022 and showed varied segmentation patterns. We establish a linkage between this decrease in confluence angle and increase in annual runoff in the study area. Expansion along the banks of the Niger, Benue, and post-confluence channels was found to be low and non-uniform, suggesting varied resistance of the banks to erosion, which is linked to the abundance of vegetation. These decadal changes in confluence planform are important for understanding river confluence dynamics in major river systems around the world.