I would imagine that freshwater animals and plants would probably die if submerged in salt water for very long. Some animals may be able to get back to fresh water but I’m sure that the whole area is polluted by the intermixing of fresh and salt water not to mention human sewer systems that must also be in the mix. Kinda yucky all around.

I just googled it and found that alligators can’t live in salt water because they don’t have salt glands like crocodiles do. They can live for short periods in salt water, though, so I would think a few days in flood water might be ok. It’s not the same as putitng the alligator in the ocean, which is obviously not diluted and would not be short-term.

Very interesting about the alligators vs. crocodiles. I remember learning differences in school, but couldn’t name them if I tried.

I googled “can alligators live in salt water” https://untamedanimals.com/can-alligators-live-in-saltwater/#:~:text=%20Alligators%20can%E2%80%99t%20live%20in%20saltwater%20as%20they,they%20do%20not%20have%20salt%20glands%20like%20crocodiles.

Alligators do live in brackish water. They do quite well, in fact. I used to work in the environmental section at a nuclear power plant on the coast. It was on a river, but only about a mile from the ocean so it was very brackish. The water we brought in to cool the plant was brackish and it had all sorts of gators in it. There were marshes next to the plant and there were some bigguns over there. Saw one that was at least 11’ long.

^^What about other animals in the waters? Will Some die off in large numbers?

If you had a fresh water lake that suddenly was inundated with salt water, it is likely that most things would not survive. Rivers and marshes are often brackish when they are close to the coast so all you are really doing is changing the salinity a little bit. Most things that live in those waters will survive, I’d think.