SIERRA VISTA, ARIZONA “For sale:  Prime Office/Retail," proclaims the sign on a mesquite flat on the outskirts of this affluent city of 47,000 people, about an hour south of Tucson near the Huachuca Mountains. It's announcing a 2,000-acre project known as Tribute, proposed by California developer Castle and Cooke and approved by city leaders six years ago. Plans call for nearly 7,000 homes and apartments, plus offices, shopping, parks and schools. The real estate bust has temporarily derailed it, but eventually up to 250 homes a year could be built. Six miles east lies the San Pedro River, the Southwest's last free-flowing major desert river. It boasts the region's healthiest remaining riparian habitat and is home to about 80 different mammals, including coatimundi and badgers, and 350 to 400 bird species.  Four million people a year visit the San Pedro, which is so closely observed by hydrologists and riparian