The Santa Cruz River is flowing vigorously near West Ina Road and for miles to the northwest even as Tucson withers in triple-digit heat.

A natural wonder in the bone-dry desert? Not exactly. The flow is effluent from wastewater reclamation facilities, but the water is so clean that fish are living in it, Pima County officials say.

The fish are non-native species apparently released in the river by people. But their thriving presence raises the possibility of one day re-establishing native fish species in the river.

“The quality of the water has greatly improved due to upgrades” at two water reclamation facilities that send treated effluent into the river, said Kerry Baldwin, Natural Resources Division manager for the county. “We are seeing improvements in many of the basic water quality factors being monitored: increasing dissolved oxygen, stable pH levels and water temperatures, and declining nitrogen and phosphorus levels.”

Baldwin said the increasingly clean water is attracting aquatic wildlife . The river lost much of that wildlife as flows diminished over the decades and less-effective wastewater treatment facilities did little to accommodate its return.

Now, “from aquatic invertebrates to fish to other wildlife, we are seeing all the signs of an improving ecological system,” Baldwin said.

He said non-native species living in the river probably include western mosquitofish and fathead minnows.

“As the system improves, it is the intent to reintroduce native species,” Baldwin said. “Native species of interest include Gila topminnow, desert sucker, Sonoran sucker and longfin dace.”

Contact reporter Doug Kreutz at or at 573-4192. On Twitter: @DouglasKreutz