By Keri Bridgwater Framed by the Superstition Mountains and swaths of giant saguaro cactus, Bush Highway in Mesa might be one of Arizona’s most scenic drives. It sweeps through 15 miles of Tonto National Forest, where Upland Sonoran Desert steadily gives way to Ponderosa pine-covered mountains just half an hour from downtown Phoenix. The lower Salt River also winds its way through here. Lined by mesquite, palo verde and acacia trees, it’s where the wild horses come to cool off, especially during the hot season when temperatures average over 100 degrees. The Tonto National Forest was establishe…