Rivers are natural entities. Yet while they occupy a more or less fixed pathway on the land, they inhabit a more fluid space in the imagination. From the early days of the American republic, the Rio Grande was a place of speculation, ripe for debate among individuals who had never seen it and never would. Well-known figures such as Thomas Jefferson, Alexis de Tocqueville, and John Quincy Adams all found the river a fertile place, but less well-known figures anchored their thoughts there, too. The power of the Rio Grande derived from its capacity to inspire reflection on the proper boundaries between peoples, nations, and races – boundaries negotiated in words but also through violence. Mexicans, Europeans, and Americans all found in the Rio a place to envision the outline of a new global order.