What did Sal in the book Walk Two Moons do in Sioux Falls South Dakota?

Sal (Salamanca Tree Hiddle) and her grandparents were traveling to Idaho to find out what had happened to Sal's mother. On their way, they traveled through Sioux Falls, South Dakota. The only event in the book that mentions Sioux Falls, South Dakota says that it was very hot in South Dakota and that Gramps took off his shirt in Sioux Falls.