Prolonged droughts can kill rivers, but Tucson’s current dry spell has actually helped the Santa Cruz River flow farther north than it had been.
Reclaimed water discharged into the river by Tucson Water’s Heritage Project is now flowing well north of Speedway. Three months ago, it was stopping between Congress Street and St. Mary’s Road, almost a mile south of where it flows now.
But after an up-and-down series of effluent releases into the river, Tucson Water isn’t putting more water there than before. The river flows farther now because dry weather has made it harder for the water to seep into the aquifer, thus prolonging its flow for longer distances, utility officials say.
For this, credit the bright green algae that populates much of the river now.
“It’s called soil clogging and is fairly common,” Tucson Water spokesman Fernando Molina said of the algae’s impact on the river’s flow.
Algae forms because the lack of rainfall allows for very little scouring of the riverbed by storm runoff, Molina said.
“Scouring” refers to the force of heavy flows of water moving the top layers of sediments, removing algae, and opening up pore spaces.
So algae forms naturally in the river channel. It clogs pores in the riverbed through which the reclaimed water normally seeps into the aquifer, Molina said.
Now the water runs farther, even though the utility is still releasing the same 800 gallons per minute of reclaimed water that it has been since September.
While the northern half of the wet section of river doesn’t contain any visible algae, algae is often found in sediments below the water, Molina said.
This algae-clogging effect has been documented nationally in a number of studies, and some scholars have characterized it as a problem for artificial recharge projects such as the one Tucson Water is conducting on the Santa Cruz.
But once more rainfall returns to the Tucson area, and “once we have some scouring of the channel taking place and the algae is removed,” the water should seep more quickly into the aquifer and not flow as far on the surface, Molina said.
In September, the city slashed releases into the Santa Cruz to their current level from 1,900 gallons per minute. It was done to prevent the aquifer from recharging so fast and so high as to potentially undercut a downtown-area landfill west of the river, which had been a concern off and on for months.
Before that cutback, the river water was flowing to about halfway between Speedway and Grant Road.
The utility started discharging reclaimed water into the river just north of 29t Street in late June 2019.
Besides the algae formation, the river also flows farther now because the weather is cooler and evaporation rates are lower, Molina said. Plus, notches that were installed in concrete during channel renovation work last spring are keeping the water centered in the channel, he said.
Historically, the Santa Cruz used to flow north of Tucson to as far as Red Rock in Pinal County, but now turns dry in northern Pima County.