What advice did John Thornton give to Hal and Charles in The Call of the Wild?

Hal and Charles, along with Mercedes, who is Charles's wife and Hal's sister, purchase Buck and the rest of a team of sled dogs from a Scotch half-breed who works for the Salt Water Mail. Although the dogs are still exhausted, the inexperienced trio loads a sled with a mountain of supplies and heads for Dawson. They are ignorant of the ways of northern travel. They take on more dogs than they can feed, set up and take down camp inefficiently, and are too weak for the rigors of the trail.

By the time they reach John Thornton's camp at White River, several of the dogs have died and the others are so tired and ill-fed that they are near death. Despite the exhaustion of the dogs, Mercedes insists on riding in the sled instead of walking alongside.

The advice that John Thornton gives to Hal and Charles is to not attempt to cross the ice over the White River. Spring has come, and the ice is melting and rotten. Thornton says, "I wouldn't risk my carcass on that ice for all the gold in Alaska." The three travelers ignore his advice. Buck can't move and Thornton cuts him out of the harness. The rest of the dog team and Hal, Charles, and Mercedes go on, and as Thornton and Buck watch, while they're crossing the river a section of ice gives way and they all drown.

What advice does John Thornton give to the Americans in The Call of the Wild and why?

After Buck and the others reach Skaguay, the dogs hope to rest, but they are sold to two men and a woman, none of whom know anything about sled dogs. Whipped and forced to go when they are nearly dead, the dogs suffer terribly.

Pike, the malingerer, who, in his lifetime of deceit, had often successfully feigned a hurt leg, was now limping in earnest. Sol-leks was limping, and Dub was suffering from a wrenched shoulder-blade.

At a stop along the way to Dawson called Five Fingers, Hal, Charles, and Mercedes feed the dogs horsehide instead of meat. The dogs become even more weakened and starved, and some new dogs die. The three buy more dogs and lighten their load, but they now have fourteen dogs, and there are more of them than can be fed. When the team finally pulls the three people into John Thornton's camp at the beginning of Spring, 

[H]e knew the breed, and he gave his advice in the certainty that it would not be followed.

Thornton tells them that only foolish people would have tried to get this far already because the ice is melting on the lake. As he predicts, though, they do not listen.

I tell you straight, I wouldn't risk my carcass on that ice for all the gold in Alaska.

Just as Thornton predicted, Hal, Charles, and Mercedes do not listen, and they try to push on. Hal whips the dogs, but even Buck can go no further. Thornton cuts Buck out of the traces, but the three people insist upon going on. True to his prediction, Thornton watches the sled, dogs, and people fall through the ice and drown in the dark water.