evidence: Liquid geography : the Yalu River and the boundaries of Empire in East Asia, 1894-1945 This dissertation examines imperial Japan's efforts to transform and control the Yalu River boundary between Korea and Manchuria (Northeast China) during the period 1894-1945. Following the Japanese colonization of Korea in 1910, the Yalu River, together with the neighboring Tumen River, formed the longest formal, non-maritime border of the Japanese Empire until its dissolution in 1945. Yet despite the Yalu's strategic significance in the imperialist politics of early twentieth-century Northeast Asia, its modern history has been surprisingly overlooked. I address this historiographical gap while making three key interventions in the larger scholarship on borders, climate history, and Japanese imperialism. First, by drawing on archival materials in Japanese, Korean, Chinese, and English, I argue that dynamic environments like the Yalu are more than mere backdrops to border histories, but co-agents of border creation and contestation alongside local human residents. A focus on what I call the liquid geography of the Yalu border shows how the transnational flow of people, goods, and the river itself exposed the limits of the colonial state even while providing the material underpinnings for further imperial expansion. Second, in analyzing the river as an agent of history, I propose a new, seasonal approach to history.