As melting water gushed off the ice in a tinseled maze of rivulets and tumbled through a gaping chasm, the hikers watched, wondered and worried. Unlike most backcountry travelers who pitch their tents along the John Muir Trail in the upper reaches of the Lyell Fork of the Tuolumne River, these visitors had not pushed on to scale the summit of Mount Lyell -- Yosemite's highest peak.