Many valleys in the southwestern United States experienced rapid and pronounced erosion at the end of the nineteenth century, yet the fragmentary historical record seldom allows a detailed reckoning of this event. The single most extensive record of arroyo-cutting in the region belongs to the Santa Cruz River of southern Arizona, where rich archives accumulated as the valley's major city, Tucson, evolved from mud-walled village to metropolis. The present study traces the development of a continuous channel in previously unentrenched segments of the Tucson Basin. To illustrate these changes, we draw on a rich photographic record of the Congress Street crossing, just a few hundred meters west of the oldest part of town.