How does Buck save John Thornton?

Buck saves John Thornton by instinctively diving into a raging river to rescue him when Thornton falls into the water during a precarious situation. Displaying his loyalty and strength, Buck swims against the current and manages to grab Thornton's arm, pulling him to safety on the riverbank. This act of bravery solidifies their bond and showcases Buck's deep attachment to Thornton, marking a pivotal moment in their relationship.