How does Augustus cray lose his legs in lonesome dove movie?

Augustus McCrae doesn't lose his legs in the Lonesome Dove movie. He loses his arm.

This happens during a Comanche ambush while Augustus and Gus are trying to make their way to the safety of the Lonesome Dove ranch. Augustus is badly injured, and his arm is eventually amputated.

The film, while loosely based on the book, takes liberties with certain aspects of the story. In the book, Augustus loses his arm after falling from a horse, not during an ambush.