In the Heart of the Canyon by Elisabeth Hyde

A group of twelve people take a two-week rafting adventure on the Colorado River through the Grand Canyon. J.T. is the head guide on his 125th trip. His assistants are Abo and Dixie. As for the tourists, Jill and Mark are accompanied by their two teenage sons. Ruth and Lloyd are on their last whitewater trip because of Lloyd's advancing Alzheimer's. Susan is with her overweight daughter--they can barely stand to be around each other. There are the singles, Evelyn (a Harvard professor) and Peter (a twentysomething who's tending to his broken heart). Lastly, there's Mitchell (who gets on everyone's nerves) and his quiet wife, Lena. The book does a great job of describing the setting and the ins and outs of rafting while also spending time on the characters' thoughts and feelings. The strangers get to deal with both the expected and unexpected on their journey.