By Robert Samuelsen

There are hundreds of non-descript sandy arroyos scattered across southern Arizona. Many of them have no names, and the ones that do are typically called “a wash.” Ironically, because of how devoid of moisture they are, they should probably be called a “dry cleaner” rather than a “wash!” About the only time they carry surface water are in the most severe thunderstorms of the summer monsoon season. Often, though, water does flow underground.
One of these local drainages has gained some recent unusual interest because it’s threatened and behaving differently than it has in decades. Named for an early settler killed by Apaches, Davidson Canyon begins in the northern reaches of the Santa Rita Mountains and flows northward to Cienega Creek and the Pantano wash. Near the confluence of Cienega Creek, there is a bedrock extrusion mound that includes a number of Hohokam mortars, a historical indicator of perennial water in times past. Water used to flow there!
Davidson Canyon is a notable wildlife corridor allowing for a safe mammalian traverse through a colorful tunnel under Interstate 10. Interstate highways are devastating to animal migration so tunnels and wildlife overpasses are important for genetic diversity. Many animals prefer to travel in the washes to stay hidden from predators and Davidson has some small springs and intermittent flows to nourish its travelers. There was also an ambitious desert tortoise with a tracking beacon that went an incredible 50 miles from Colossal Cave Mountain Park to the Santa Ritas and back by going through Davidson Canyon.
The headwaters of Davidson Canyon start at the controversial Rosemont mine site. More than a decade ago, I signed up for a tour of the copper project to learn about the open pit mine and its impact on the ecosystem. I had extensively explored the area’s mine holes, ghost towns, and the abandoned Rosemont smelter beforehand so I knew the area hadn’t been pristine wilderness for a century. The tour guide described the company’s environmental mitigation plans including the description of an underground geological dike protecting downstream watershed contamination. It all sounded so good, yet we all know the environmental history of mining.
The Cienega Creek watershed flowing underground in the Pantano wash provides about 20% of the City of Tucson’s water, and close to 100% of Vail’s water. Contamination in Davidson Canyon, being a notable tributary, could impact the drinking water of a million people. With the threats of development, getting baseline data of Davidson’s surface flow is really important.
Last summer, we had an incredible monsoon season – almost double our normal rainfall. Through the unusually dry winter and spring and continuing enduring impact of the worst drought in 1,200 years, Davidson Canyon uncharacteristically continued to flow after the summer rains! When I was measuring Cienega Creek flows and taking water samples in early June, incredulously Davidson was still flowing above ground strong! And even more surprising, based on the isotope analysis of its water, the water flowing today is ancient water. Mountain-Block Recharge, the subsurface inflow of groundwater to lowland aquifers from adjacent mountains, is pushing ancient water out from the bottom of an upstream hydrologic dome. More succinctly, last year’s rain is replacing dinosaur water in the Rosemont/Davidson aquifers and watershed. How cool is that!.

Rob Samuelsen is an executive and adventurer supported by his long-suffering but supportive wife!

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