Bighorn sheep know the Cottonwood Spring. So do savvy backpackers who use its reliable water to lighten their load on the 30-mile Cottonwood Canyon/Marble Canyon Loop, or to linger and explore side canyons. One part of this loop, navigating the upper connection between Cottonwood and Marble, is a task for experienced route finders with detailed topo maps. Others should stick to an out-and-back course in either Marble, known for its deep narrows section, or Cottonwood, with its namesake shade trees. Both are memorable trips. Be prepared: Access may require 4-wheel drive.