Buffalo Storm by Katherine Applegate, illustrated by Jan Ormerod
The story of a girl whose family goes west in a covered wagon. She is very close to her grandmother- they are both afraid of storms- who gives her a quilt to remember her by. During the trip, she sees a bison calf who is caught in between some rocks, she frees the calf and then sees a stampede of bison that reminds her of a storm.
The story would be a good one to use with ages 6 and up - there is a lot of text, so for independant readers I'd say 2nd grade, early 3rd grade level. I love Jan Ormerod's artwork and this one is softer in its style than her other works.