Hopping from one rock to another, Sue Ann Arens led her two boxers one recent afternoon up and down the uneven slopes of Folsom Lake, which years of drought have stripped bare. Their daily walks along Sacramento’s backyard reservoir had become a hike through the lake bed.

Well-worn trails threaded through clusters of brush in areas usually covered by water.By the next afternoon, Arens had to change course. A morning storm had submerged part of her trail. In the distance, water rippled over an island that had surfaced last year when the lake hit bottom.