“As the last of the night cupped the river in its hands, Petschek knew in his heart that it was simply impossible to come away from an extended encounter with the greatest canyon on earth without acknowledging that human life, despite its frailty and insignificance, represents a bestowal of grace for which one has done nothing whatsoever to deserve—and that for this reason, the river world vibrated with a harrowing beauty whose principal dividends were gratitude and joy.”