The Desert Hawks, The

From back of book:

Zac Cobb, an agent for the Wells Fargo Company, is on assignment in northern Arizona to track down the bandits robbing army payrolls and murdering the armed escorts. The tension mounts when the habits of one of the men Zac is tracking become disturbingly familiar. The leader of the outlaw gang turns out to be his brother Julian, a man driven by bitterness and cruelty more than the pain from losing an eye and arm during the Civil War.

With the help of an Apache scout, Zac closes in on the outlaws at the banks of the Colorado River where a ferryboat is about to cross. As a stagecoach carrying innocent passengers and ammunition for the army inadvertently converges on the same crossing, a band of marauding Indians led by a Yavapai war chief attacks the stagecoach.

Thrust together for survival, Zac and his brother and the others on the ferryboat realize that their only hope of escape is to cut the boat free from its ropes and run the rapids that race down into the massive Grand Canyon. But the peril of the careening boat through the churning rapids cannot compare with the life-and-death battle that rages between the two brothers.