What does the destruction of the “naturally” created raft by the “industrially” created steamboat symbolize in Huck Finn?
The raft, whether Mark Twain would have liked it or not (see his note in the introduction of the book), comes to symbolize man's relationship with nature and the concept of freedom, away from the constraints imposed by civilization. In Huck's situation, the raft gives him the freedom to go where he wants to and be what he wants to be. Huck enjoys his freedom; he loves being out in nature even when he has to endure pain.
In addition, Jim also finds freedom on the raft. It literally becomes a vehicle for him to escape white oppression. The raft offers both him and Huck the opportunity to bond. They form such close ties with one another that each is willing to make sacrifices for the other, sometimes at considerable risk.
The steamboat is a symbol of civilization, dominance and power. As such, it is a threat to nature and freedom. When Huck describes its collision with their raft, he says the following:
She was a big one, and she was coming in a hurry, too, looking like a black cloud with rows of glow-worms around it; but all of a sudden she bulged out, big and scary, with a long row of wide-open furnace doors shining like red-hot teeth, and her monstrous bows and guards hanging right over us. There was a yell at us, and a jingling of bells to stop the engines, a powwow of cussing, and whistling of steam—and as Jim went overboard on one side and I on the other, she come smashing straight through the raft.
The diction in the above extract makes it clear what the steamboat represents. The vocabulary alludes to a frightening monster, hell-bent on destroying and/or devouring everything in its path. In Huck's contention, this is exactly what being "sivilized" means. To him, civilization, with all its rules and conventions, sucks the very soul out of one.
Both Huck and Jim attempt to escape from civilization and the raft provides them the opportunity to do so. When they are on the raft, they have only the natural to contend with. They are free of the hypocrisy that society represents, and they are away from the unjust laws that it so arrogantly defends. The two feel liberated from the norms that harshly punish trivial oversights and ignore the most severe abuses, such as, for example, the utter contempt for Jim's humanity.
Fortunately, though, all is not lost. Jim manages to salvage the raft and fix it. Once the two have reclaimed their lost property, they resume their journey, enthralling us with their exploits. This, in itself, is symbolic of the fact that civilization can save itself; a harmonious existence between man and nature is, after all, possible. It is, furthermore, an indication that nature has the power to recreate itself and survive, in spite of civilized man's best efforts to destroy it.
You have just about answered this question yourself by the way you ask it. This incident symbolizes the way in which industry and technology are taking over the world in Twain's time.
During the time that Twain was writing, the US was becoming much more industrial and technologically advanced. This development was changing society in a major way and was in the process of killing off a fair amount of the rural society that Huck lives in.
I think that there is one other thing going on here. In this book, society is the problem and Huck's own natural values are really better than society's values. By having this accident happen, Twain may also be pointing out how society's values are trying to crush Huck's natural values.