The confluence of two ephemeral washes

Sinclair Wash flows from Rogers Lake (about 6 miles away). The Rio de Flag (in the culvert under your feet) flows from the west side of The Peaks (the big mountains you see to the north, a mile higher in elevation than this location). The Rio de Flag has flowed in this channel since about 1910, when it was diverted from downtown Flagstaff. Both streams flow during spring snowmelt (8 weeks in a good snow year) and for a few hours after each summer thunderstorm. In the early 2020’s a flood control project will put the Rio’s flow into a buried concrete box at Frances Short Pond to emerge 3 miles later. When the flood control project is complete, this Rio channel will be dry, but the smaller Sinclair Wash flows might still be able to support the willows and other riparian plants downstream.

The City of Flagstaff is currently (2021) considering releasing reclaimed water just west of S1, which would keep Sinclair Wash flowing almost all year except many days in May and June when golf courses guzzle our reclaimed water. For the next mile downstream, the north canyon wall is Black lava and the south canyon wall is pale limestone. Until about 1 million years ago a predecessor of the Rio de Flag flowed west to east approximately along what is now Butler Avenue. Then the Dry Lake Volcano, 4 miles west of Flagstaff erupted, filling the stream with a type of lava (andesitic basalt) with a thick, sticky consistency (highly viscous). So viscous that the weight of the thick lava flow pushed some of the lava upslope into the adjacent floodplain and side channels. The basalt you see at each of these points are the uphill “toes of the flow.” Over time, Sinclair Wash carved a canyon between the basalt and the limestone, mostly by cutting away the softer limestone.

Holm, R.F., 2019, Geology of Flagstaff and Geologic History of Rio de Flag, Northern Arizona with Trail Guides to Geology along Rio de Flag. Arizona Geological Survey Down-To-Earth #23, 70 pages.