The phrase ‘he’ll do to...
The phrase ‘he’ll do to ride the river with’ comes from the trail drives. Often a river needed to be crossed is suddenly flooded with swift currents from flash floods many miles away. These can come up quite suddenly and a herd caught in one is in a lot of trouble.
A cowboy who would go into such a whirling watery mess to help another rider is risking his own life to do so. Hence, if a cowboy will do that, he’s not only brave, but strong and loyal, a good friend.
So if there’s someone in your life who you would trust with anything and everything, he’ll ‘do to ride the river with’. :)