The Dead Lands by Benjamin Percy

As far as these few survivors know, from their vantage behind the high walls of The Sanctuary, “the observeable world” has perished. Beyond their city’s walls spans a land of unknowable nightmares, a wasteland seemingly devoid of humans but haunted by new monstrous mutations, arid with frightful radiation.
In The Dead Lands, a fatigued group clutches to a fading sense of civilization (and deluded patriotism) in a future St. Louis, Mo.: the only place, it is assumed, to have successfully staved off a new strain of flu that, after 150 years, has wiped out the entire human species.
That’s until a mysterious, pint-sized traveler named Gawea, with piercing, pitch-black eyes, wanders to the Sanctuary, coming from the West and speaking of an outpost in Oregon, centered around a mystical shaman-like luminary calling himself Aran Burr. This catches the attention of the shrewd and introverted curator of the Sanctuary’s museum, an essentially ostracized man thought to be a wizard himself, named Lewis Meriweather.
Sound familiar? Well, we soon meet one of the Sanctuary’s top sentries, a cunning, hard-edged young woman named Mina Clark. She doesn’t know what to think of this strange, young traveler and her claims of fresh water, crops and civilization, out there in “the dead lands…,” but she certainly doesn’t agree with the villainous mayor’s harsh decision to outright execute this seemingly harmless girl.
Eager to evade the oppressive regime of this wicked, Prince John-ish mayor and his comparably nefarious Sheriff of Nottingham-esque lackey, Clark implores Lewis to join her. Clark, her brother, a doctor and a handful of other bold travelers, leave on a daring mission—not just to save the girl, but to then breakout of this prison of a “sanctuary” and head West.
This is where it gets supernatural. Because it becomes certain that young Gawea is “different,” eventually, we discover that Lewis also shares an extraordinary ability of hers, one that has caused Lewis to experience these feverish, lucid-dream visions of Burr, silver beard and all, beckoning him westward.
“Magic is just a word people use for what they can’t understand…” Gawea says, both defending and encouraging Lewis’ burgeoning powers.