What happened to John Thornton at Eorty mile Rapids? Chapter 6

Thorton's boat tipped and he fell in.... Buck saved his life.

The boat flirted over and snubbed in to the bank bottom up, while Thornton, flung sheer out of it, was carried down-stream toward the worst part of the rapids, a stretch of wild water in which no swimmer could live.

Buck had sprung in on the instant; and at the end of three hundred yards, amid a mad swirl of water, he overhauled Thornton. When he felt him grasp his tail, Buck headed for the bank, swimming with all his splendid strength.