| Rillito River | |
| Name Etymology: | Spanish word meaning "Little River" |
| Pushpin Map: | Arizona |
| Subdivision Type1: | Country |
| Subdivision Name1: | United States |
| Subdivision Type2: | State |
| Subdivision Name2: | Arizona |
| Subdivision Type5: | City |
| Subdivision Name5: | Tucson, Arizona |
| Length: | 12.2miles |
| Source1: | Confluence of North and South Forks |
| Source1 Location: | Pima County, Arizona |
| Source1 Coordinates: | 32.2686°N -110.8753°W[1] |
| Mouth: | Santa Cruz River |
| Mouth Location: | Pima County, Arizona |
| Mouth Coordinates: | 32.3136°N -111.055°W |
| Mouth Elevation: | 2195feet |
| Tributaries Left: | Tanque Verde Creek |
| Tributaries Right: | Pantano Wash |
The Rillito River (pronounced as /es/; Spanish "Little River") is a river in Pima County, Arizona. 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 12.2mile away.[2] The Rillito River Park runs along the north and south banks of the river from Interstate 10 to North Craycroft Road.[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. The loss of vegetation led to increased erosion of the river banks during flood events, which in turn led to a widening and straightening of the river channel.
Today, the Rillito is an ephemeral river that carries water only during floods or in response to snowmelt. In the late 20th century, as a flood control measure, many segments of the channel's banks were stabilized using soil cement to reduce erosion and prevent the water from overflowing the banks and damaging property.