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River processes and patterns are affected by changes in the watershed driven by natural and human‐induced causes. A sudden pattern alteration from a "white river" (bare soils) to a "green river" (vegetated) influences riverine biodiversity and can increase flood risk. Despite these significant impacts, knowledge on the triggers that kickstart feedback exacerbating changes in bio‐geomorphic patterns is insufficient. In this study, we collected and analyzed detailed monitoring data on a sandy, hydro‐morphologically active, and monsoon‐driven river in Korea. The surface area covered by vegetation has been increasing; this increase intensified after the 2014–2015 drought, which provided a window of opportunity for vegetation establishment. During the drought, pioneer vegetation densely colonized bare sandbars and temporarily exposed riverbed. Despite partial rejuvenation by several subsequent floods, succession to woody vegetation occurred, resulting in a stable vegetation cover. Narrowing and incision of a low‐water channel occurred, and secondary channels formed inside the floodplain. The results of this study show a rapid bio‐geomorphic alteration triggered by the shifts in flow regime in a river primed by human‐induced changes. Furthermore, modified monsoon‐driven rivers might be on the brink of similar bio‐physical alterations triggered through shifting flow regime following climate change, leading to increased flood risk and impacts on endemic biodiversity. [ABSTRACT FROM AUTHOR]