Monsoon floods rejuvenate Santa Cruz River ecosystem

After one of Tucson's driest years on record in 2020, recent flooding in the Santa Cruz River spells hope for a recovering riparian ecosystem and the water table lying beneath. 

Early last week, the largest sustained flow of water seen in the Downtown area of the Santa Cruz River in about five years rushed over the riverbed, creating a muddy, urban oasis for desert life. 

Such floods "help deliver important minerals and organic material, they help maintain the structure of the aquatic habitat (pools and riffles, etc.), they help reduce the abundance of non-native invasive species, and they help open space and reduce competition among native species," Michael Bogan, an assistant professor of aquatic ecology at the University of Arizona, said in an Instagram post.

According to Luke Cole, the Sonoran Institute's associate director for resilient communities and watersheds, flooding also breaks up the sediment on the river's floor, allowing more water to break through the surface and soak into the ground.

This helps with one of the ultimate goals of the Santa Cruz River Heritage Project: recharging the aquifer. 

"When you don't have scour, that prevents surface water from percolating down into the riverbed and into the groundwater," Cole said.

In the absence of occasional heavy rains and flooding, the main source of water feeding the river is the Santa Cruz River Heritage Project, a Tucson Water effort which adds 2.8 million gallons of treated effluent to the river daily. While this has rejuvenated pockets of the Santa Cruz near Downtown, it cannot harness the same ecological power as major flood events, Cole said. 

As climate data shows Southern Arizona becoming warmer and drier, important flood events could decrease. 

"We are still very much in the thick of a 21-22 year drought," Cole said. "This is a sip of water during a trek across the desert." 

During major flood events, Tucson Water does not stop discharging water into the Santa Cruz, according to Fernando Molina, a spokesman for the city agency.