Is this a run-on sentence? "It continued to rain until the river overflowed many people had to be evacuated from their homes.

Yes.

Yes, the sentence "It continued to rain until the river overflowed many people had to be evacuated from their homes" is a run-on sentence. It combines two independent clauses without proper punctuation or conjunctions to join them.

To fix the run-on sentence, you have a few options. One way is to separate it into two separate sentences:
- "It continued to rain until the river overflowed. Many people had to be evacuated from their homes."

Another option is to use a coordinating conjunction (such as "and" or "but") to join the two independent clauses:
- "It continued to rain until the river overflowed, and many people had to be evacuated from their homes."

Finally, you could also use a semicolon to separate the two independent clauses:
- "It continued to rain until the river overflowed; many people had to be evacuated from their homes."

Each of these options provides clarity and corrects the run-on sentence by properly separating the two independent clauses.