The Mekong River Basin experiences frequent large-scale floods driven by intense monsoonal precipitation and diverse tributary interactions, ranging from independent flash floods and anthropogenic dam operations to wetland buffering and bidirectional backwater effects. Effective water resource management, central to the United Nations Sustainable Development Goals 6 (Clean Water and Sanitation) and 13 (Climate Action), requires robust observational infrastructure to monitor flow dynamics. However, the current in-situ gauge network is primarily situated along the mainstream with sparse monitoring of tributaries, leaving critical parts of tributaries largely unmonitored. This study demonstrates the role of Surface Water and Ocean Topography (SWOT) satellite mission data in bridging observational gaps in 12 tributaries (varying in width from 15 – 600m) that connect with the mainstream river. Importantly, validation studies along tributaries that are relatively narrow and theoretically below the operational threshold for SWOT water level measurements was conducted. Results demonstrate the capability of detecting flash-flood events and anthropogenic flow alterations driven by dam operations, while also capturing the natural seasonal connectivity between tributaries and the mainstream river. By extending observational capability beyond the mainstream, a sustainable, space-based water level monitoring workflow can be established to managing water resources in regions with limited in-situ monitoring.