There's no way to understand the Tucson piece of the Santa Cruz without understanding the whole river.

Troubled Waters 

The way Larry Marshall sees it, there's no way to understand the Tucson piece of the baked Santa Cruz without understanding the whole river, from its headwaters in the Santa Rita Mountains to its endpoint at the Gila River.

"So many people think the river goes from Valencia to Ina," he says. "At Rio Nuevo you're looking only at one part of a whole river system -- There's no way you can control a short segment of the river without making modifications upstream."

Which is why we're upstream, down in Nogales, standing in a bird-loud glade. We're gazing at the beleaguered river's last cienega, or marsh. The weather has alternated all morning between cloudbursts and sunlight, but right now the sun glints off the marsh's crystal waters, lighting up the cattails, glancing off the trees. Hundreds of birds are chattering overhead.

"This is the only natural cienega left along the Santa Cruz," laments Marshall, a Berkeley-trained Ph.D. in geology, paleontology and evolutionary biology, and a member of the Santa Cruz River Alliance. This surviving marsh could serve as a model for Rio Nuevo, which proposes to restore the historical Tucson cienega that once watered the base of A Mountain.

Marshall, an indefatigable lover of the river, has proposed a car tour to its high points. We start at Nogales and end in Tucson, following the river on its northerly journey.

Geology, he says, accounts for the way the river, even in the abundantly watered days of old, ran above ground in some places and underground in others. Between Nogales and Canoa Ranch, he notes, "the river runs on bedrock," a shallow surface that pushes the water to the earth's surface. But that underground surface changes dramatically at Canoa, south of Green Valley.

The bedrock gives way to soft, porous sediment that runs deep. The northbound river waters drop into this underground chamber in a giant waterfall, pouring into the Tucson Basin, the aquifer that's long given the city its drinking water.

"Historically -- the water stopped at Canoa," Marshall says. "There were 30 miles of no water." But travelers heading north noticed that a ribbon of green continued on the surface, the trees and vegetation drawing water from under the ground.

At Martinez Butte, just south of San Xavier del Bac, the water bubbled up again in perennially reliable springs. The mission was built in the curve of the river here, and a fairy-tale forest of giant mesquites stretched for miles. Another spring watered the marsh at what's now called A Mountain, and the early explorers recorded a crystal-clear creek at Tucson.

Between San Xavier and Tucson, Marshall says, "Sometimes there was water, sometimes not. At times of drought, it was dry between Martinez Butte and A Mountain."

North of Tucson, a "nine-mile watering hole" sprang up at Cañada del Oro Wash. Beyond there was the dreaded "90-mile desert," where the waters went underground until the Santa Cruz's confluence with the Gila River.

Today the troubled waters have deviated far from their historical course. A Nogales treatment plant pours treated sewer water into the Santa Cruz, giving this southerly stretch more water than it had historically. But the river there is shallow and wide, lined with trees, looking much as it did for centuries. We sneak down a private ranch road to take a peek, and find a satisfying creek rushing past thick stands of trees. Flood debris hangs in the branches overhead, testifying to the natural cycle in which the river overflows periodically.

From here, the Santa Cruz flows in picturesque fashion on past the Mission at Tumacacori, which was built on its banks, and the presidio at Tubac. (The Tohono O'odham word "bac," giving a name both to Tubac and San Xavier del Bac, means "where the water comes out of the earth," according to UA researcher Barbara Tellman.)

But the closer we get to Tucson, the worse the river looks. North of Tubac, a poorly designed bridge constrains the formerly shallow river. For the first time we see a cut-out channel and eroded banks of the kind so familiar in Tucson. By the time we reach Pima Mine Road, at the southern edge of the Tohono O'odham Nation, the dried-up river is deeply and permanently etched into a wide ditch. Ugly, cracked-dirt banks 20 feet high rise up on either side. From here on to the Rio Nuevo, Tucson's birthplace, the river is dry, dusty and bare.

A forlorn sign on Pima Mine Road inadvertently announces the devastation to come. It's positioned above the desiccated river channel near the mission, once the place of the waters. Neat white letters spell out: "Santa Cruz River. No Fishing."

No fishing? No kidding.

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