Final answer:
The given sentence is correctly punctuated without the need for additional commas because it contains a coordinating conjunction 'and' joining two closely related independent clauses.
Step-by-step explanation:
The correct punctuation for the sentence "The South Fork dam broke and the city of Johnstown was flooded" is already accurate as it stands. It contains a coordinating conjunction 'and', which joins two independent clauses; hence, a comma is not necessary before 'and'. In English punctuation, this sentence structure is correct because both clauses ('The South Fork dam broke' and 'the city of Johnstown was flooded') are closely related in meaning and each can stand as a sentence on their own.