Visitors to the Crystal Bridges Museum of American Art in Bentonville, Arkansas, leave appreciative notes on a glass wall near the entrance.

"Thanks Alice!" reads one. "Merci Alice Walton, pour la vision!" reads another.

Wal-Mart Stores Inc. heiress Alice Walton founded Crystal Bridges in 2011 in a wooded ravine next to her childhood home, supplying dozens of paintings from her personal collection. Bankrolled by more than $1 billion in donations from her family, the museum attests to the Waltons' generosity and vast wealth. It's also a monument to their skill at preserving that fortune across generations.