This chapter systematically reviews global and regional literature on runoff alteration and delta connectivity. It is structured around two themes: the impacts of climate variability and human activities on runoff, and upstream–downstream hydrologic linkages. The review synthesizes diverse methodological approaches and finds that runoff responses vary greatly with basin scale, intervention type, climate regime, and physiography. Critically, the chapter identifies gaps: tropical monsoon basins like South Koel are under-studied, and few studies link upstream changes to downstream deltaic effects. These gaps justify the book’s region-specific, integrated focus.