My Abandonment by Peter Rock

From www.goodreads.com:

A thirteen-year-old girl and her father live in Forest Park, the enormous nature preserve in Portland, Oregon. There they inhabit an elaborate cave shelter, bathe in a nearby creek, store perishables at the water’s edge, use a makeshift septic system, tend a garden, even keep a library of sorts. Once a week, they go to the city to buy groceries and otherwise merge with the civilized world. But one small mistake allows a backcountry jogger to discover them, which derails their entire existence, ultimately provoking a deeper flight.

Inspired by a true story and told through the startlingly sincere voice of a young narrator, Caroline, Peter Rock's My Abandonment is a riveting journey into life at the margins and a mesmerizing tale of survival and hope.