The Four Legal Repairs for a Fused Pair

Two clauses are independent when each could stand alone as its own sentence. Fuse them without the right punctuation and you get a run-on; fuse them with just a comma and you get a comma splice. Both are the same underlying problem: the boundary between the two sentences was never marked. The fix is always one of four moves, because each move signals a different relationship. A period says these are two separate ideas. A semicolon says they are separate but closely related. A comma plus a conjunction (and, but, or, so, for, nor, yet) says separate, related, and here is the logical connector. A colon says the second clause explains, defines, or delivers on a promise made by the first.

Apply all four to 'The dam held. The floodwaters rose another meter.' You could write it as two sentences, or 'The dam held; the floodwaters rose another meter,' or 'The dam held, but the floodwaters rose another meter,' or 'The dam held: the floodwaters rose another meter' if you mean the rise was the direct consequence worth spotlighting. Choosing between semicolon and comma-conjunction is partly about whether a conjunction is already sitting in the sentence; if 'but' is there, you need the comma before it, not a semicolon stacked on top of it.

A dash can informally join two independent clauses too, but it is not one of the four tested repairs, because its job is to signal an abrupt break or emphasis, not a specific logical relationship, so it never has one correct required use the way the colon does. And the moment one side of the pair stops being independent, such as a time clause starting with 'after' or 'because,' none of the four repairs apply, since there is no fusion to fix at all.