Urban flooding is a major socio-economic challenge in many cities of the Global South. Inadequate and unequal urban planning often excludes the poor majority, while rapid urban change makes it difficult to obtain reliable, up-to-date data for flood management. This study investigates changes in the river channel and informal settlements along the Msimbazi River in Dar es Salaam, one of the world's fastest-growing cities, which faces increasingly severe annual flooding. We use high-resolution satellite imagery, open elevation data, and a relative elevation model approach to map flood-prone areas and analyze flood-induced transformations between 2013 and 2022. Our findings show that even a small change in the Msimbazi River's water level leads to significant shifts in flood zones, with risk areas evolving during each flood event. During the study period, flooding also triggered substantial geomorphological changes in areas not included in existing flood risk maps. These changes would have remained undetected without high-resolution imagery or field investigations. Informal settlements underwent considerable spatial shifts following floods, disproportionately affecting the most vulnerable populations. Informal settlements were also expanding further away from the city center, where flood risks are primarily geomorphological rather than driven by overbank flooding and thus overlooked in the local flood risk assessments. The results of this study emphasize the need to integrate geomorphological insights and vulnerability indicators into flood risk assessments and to engage civil society in the process. Additionally, they highlight the importance of making high-resolution satellite data widely accessible, ensuring that its benefits extend beyond wealthier groups.