When Hal and Charles and Mercedes show up at John Thornton's camp on the White River, he gives them a warning. He tells them that they really should lay over instead of trying to continue on down the river. Thornton tells them the ice is no longer strong enough to risk sledding on it.
Hal and Charles, of course, do not listen and they push their team onwards. Unfortunately for them, Thornton was right and they and their dog team drop in to the water and die.
Luckily for Buck, Thornton has taken him from Hal and Charles because he could not stand the way the men were treating Buck.
Good answer pohnpei397!
My advice: REad the Book!