Bank erosion hotspots in the Msimbazi River, Tanzania

Abstract

In rapidly growing urban areas, river dynamics have become major hazards, endangering vulnerable populations and their environments. The Msimbazi River Basin in Dar es Salaam, Tanzania, demonstrates these challenges, where unplanned urban expansion, deforestation, and infrastructure development have influenced river morphodynamics. Although prior studies have addressed flood risks, sedimentation, and bank erosion, an understanding of the river’s migration patterns, widening trends, and key drivers at the full catchment scale has been lacking. This research applies a descriptive approach combining manual riverbank tracking from satellite imagery, identification of potential morphological change drivers, and hotspot mapping based on hazard severity and exposure susceptibility. From 2007 to 2024, the Msimbazi River experienced significant lateral migration and localized widening, with notable episodes of instability after 2017. These changes are primarily driven by land cover transformations, such as urbanization and deforestation, which have led to gradual shifts in river morphology, rather than being the result of changing precipitation patterns. In contrast, abrupt and localized changes, particularly after 2017, appear to be associated with infrastructural developments like the Standard Gauge Railway, as well as the coincidence of the El Niño phase with the cyclone season, which contributed to elongating the wet seasons. Findings reveal that river migration rates and widening trends fluctuate considerably, often doubling or tripling during periods of rapid change, with the highest vulnerability observed in the upper catchment. By identifying zones where hazard severity and exposure intersect, this study provides a framework for prioritizing mitigation in the Msimbazi Basin and other urban rivers in rapidly eveloping regions.