Environmental History Conference Offers Some Clues. By Margaret Regan SMACK IN THE middle of Phoenix, hemmed in by the city's blacktop and fast-food joints, is a surprising riparian oasis. Called Tres R the confluence of the Salt River and two lesser-known rivers. "Wood ibises and bobcats have been sighted there," says Diana Hadley, an environmental historian at the Arizona State Museum. "It's like a jungle; it's just beautiful." Nothing, in fact, like Tucson's own parched and punished Santa Cruz, the defunct waterway west of downtown that some local optimists are trying to revive. But water-deprived Tucsonans have a chance to learn how to pull off successful water projects like Tres Ríos