evidence: This paper uses disputes over islands in the early twentieth-century Yalu River to examine how river environments critically shape border conflicts. Processes of riparian erosion and sedimentation along this historical border between China and Korea, accelerated by upstream deforestation, helped transform submerged sandbanks into islands known as Hwangch’op’yŏng. After Japan’s occupation of Korea in 1905, these islands in the lower Yalu delta—and the valuable reeds that grew on their marshy surface—became an unlikely battleground between Chinese and Korean reed harvesters, Japanese "continental adventurers," and competing Japanese and Chinese empires. With the contemporary Sino-North Korean border still a geopolitical hotspot, it is imperative to understand the historical as well as ecological forces that shaped the contested geography of the Yalu River boundary. Such an understanding, this article argues, shows how both Japanese imperial violence and riparian sedimentation contributed to Hwangch’op’yŏng’s battleground status and its transformation, by the end of the twentieth century, into a Korean exclave on the Chinese bank of the Yalu River. [...] For a detailed discussion of implications of Narita’s 1906 survey of the lower Yalu, see O (2017).