It flows from east to west across the northern boundary of the City of Tucson from the confluence of Tanque Verde Creek and Pantano Wash to the Santa Cruz River 12.2 miles (19.6 km) away.

[3] Prior to colonization by European settlers, much of the Santa Cruz valley was filled with riparian habitats, including numerous zones along the banks of the Rillito River.

As recently as the late 19th century, the river was a perennial stream lined with trees and dense vegetation such as cottonwoods, willows, and mesquites.

[4] However, due to increased pumping of groundwater for irrigation projects to support agriculture and urban development, the river eventually dried up and much of the riparian habitat was lost by the mid-20th century.

[4] This article incorporates public domain material from the United States Army Corps of Engineers