On the surface, new litigation over the San Pedro is pitting the Southwest's last major free-flowing river against the future of Sierra Vista and its environs.

Looking deeper, the lawsuits pit two longstanding legal traditions about water in Arizona against one another. The issue is: Can Arizona control groundwater pumping that could damage a river when the federal government has rights to its flow?

Those differences are so acute that a former longtime state water official, Herb Guenther, sees the dispute as a legal train wreck waiting to happen.

The U.S. Bureau of Land Management and two environmental activists filed separate Superior Court lawsuits this month, seeking to overturn a state ruling that developers of Tribute, who would build 6,900 homes and apartments in Sierra Vista, have an adequate 100-year water supply. That ruling is needed for any new Cochise County housing development to break ground.

On the side of developer Castle & Cooke is the state's legal tradition, dating back more than a century, of regulating groundwater and surface water separately, even though most hydrologists say they're related.

On the side of those filing suit is the BLM's water rights to the river, established by Congress in 1988 when it created the 40-mile-long San Pedro Riparian National Conservation Area. It stretches north from the Mexico border to just south of St. David. BLM wants the courts to rule that its water rights take precedence over the developer's water rights.

"This conflict has been brewing for a long time, and is just coming to a head," said Guenther, the Arizona Department of Water Resources' director from 2003 to 2011 who now works as a consultant.

Tribute's more than 1,900-acre housing development, north of Buffalo Soldier Trail and east of Arizona 92, will serve all growth predicted for the Sierra Vista area for 30 to 40 years, said Castle & Cooke. The company has invested $7 million in the project, plus its land purchase cost.

For nature lovers, the San Pedro is a ribbon of green, lying 4 1/2 to six miles east of where Tribute's wells would pump groundwater. Drawing hundreds of bird species and more than 100 other mammal, reptile and amphibian species, the river is "a biological treasure chest," one of the lawsuits says.

"This is the case we've been waiting for a long time," said activist Robin Silver, a river-area landowner and plaintiff in the case. The other suit was filed by Tricia Gerrodette, a longtime Sierra Vista environmental activist.

"There's a set of facts very threatening for the river," Silver said, citing a significant decline in the San Pedro's flows since 1900, as seen in U.S. Geological Survey studies.

People's property rights are at stake, too, countered Gail Griffin, a state senator who represents Sierra Vista and is a real estate broker. "Water rights are property rights. If BLM were to win, it would be devastating to people in the area."

The Arizona Department of Water Resources ruled in April, following an administrative law judge's hearing, that it lacks authority to limit groundwater pumping to prevent impacts to the river. The state doesn't consider such impacts a valid legal standard for an adequate water supply ruling, said Andrew Craddock, director of the agency's adequate and assured water supply programs.

The department and the law judge also noted that Cochise County voters in 2010 killed a ballot proposal to create a regional water district with clear powers to limit pumping to keep the river flowing.

"Consequently, consideration of the impacts of … pumping on the San Pedro River clearly would be inconsistent with legislative intent," state law judge Thomas Shedden wrote in his ruling in March.

A BLM official counters that the bureau has a responsibility to protect its water rights.

"The (state water department's) decision has the potential to diminish streamflows on the San Pedro. That's obviously a concern to us," said June Shoemaker, chief of BLM's Arizona renewable resources branch. "Congress designated the conservation area to protect streamflows and their associated values. The decision didn't consider the impacts on the river flow."

In their suits, BLM and environmentalists noted that the U.S. Geological Survey has concluded that during Tribute's first 50 years, up to 30 percent of its pumped water would have otherwise gone to the river. Environmentalists said the river's typical June base flows - its only consistently dependable flows - are much lower than the water rights BLM is claiming, making the river already oversubscribed.

A former Interior Department solicitor - who helped draft water rights language in the law setting up the riparian conservation area - said he thinks the feds will probably win but will need a State Court of Appeals or Supreme Court ruling to prevail. "The state and the county want the economic and other benefits of a San Pedro National Conservation Area but they don't want to protect the river - which is the whole basis for the area," said the former solicitor, John Leshy, a University of California - Hastings law professor. "Apparently, it's still true in Arizona that land development trumps all, even if it scoffs at senior water rights and destroys a world-class resource."

But for BLM to win, it must show with credible computer modeling that Tribute's pumping will reduce the river's flow enough to compromise federal water rights, said Robert Glennon, a University of Arizona law professor.

"I don't know that it needs to be a particular subdivision that makes the case but at some point in time (one development) is going to be the straw that breaks the camel's back," said Glennon, a prominent expert on water issues. "You can't continue to have huge developments going in when the river is fragile to begin with."

Disagreeing, Griffin, the state senator, said the city can continue to have growth and a healthy river at once. She pointed to Fort Huachuca's longstanding conservation efforts and Sierra Vista's plans to increase water conservation. Real estate brokers are helping with water conservation by retrofitting fixtures when they sell existing homes, she said.

"I don't believe BLM is managing the river properly now. It's gotten thick and unmanaged now with cottonwoods," Griffin said. "If there's a fire down there, the whole thing might burn up. No use is not wise management."

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Contact reporter Tony Davis at tdavis@azstarnet.com or 806-7746.