Describes how the rivers of Japan are both hydrologically and historically dynamic. Today, these waterways are slowed, channeled, diverted, and dammed by a myriad of levees, multiton concrete tetrapods, and massive mutipupose dams. In part, this intensive engineering arises from the waterways falling great elevations over short distances, flowing over unstable rock and soil, and receiving large quantities of precipitation during monsoons and typhoons. But this modern river regime is also the product of a history that narrowed both these waterways and people's diverse interactions with them in the name of flood control. Neither a story of technological progress nor environmental decline, this history introduces the concept of environmental relations as a category of historical analysis both to explore these fluvial interactions and reveal underappreciated dimensions of Japanese history.