im no expert but i think the fish kills are mostly from two reasons, low oxygen water from dead swamp water draining out into the waterways as floodwaters receed and the second is salt water intrusion being pushed into predominantly fresh water tributaries that kill less hardy species and those decaying fish and dead vegitation clogging the water bottoms deplete the oxygen

in salt water areas the waterways are way way more open and less likely can effect it enough to change the oxygen levels or salinity enough to harm fish.

It happens in saltwater also but it's mostly in areas where the water sits and gets bad

I helped a friend in point a la hache marina with his boat sat sun and mon and everything in the marina was dead and also in the back levee canal they had redfish sheepshead trout and everything else floating all over the banks of the back levee canal the only live fish we seen was garfish

The scenario drew describes is more what happens in shallow saltwater than fresh. When we have very dry summers like this one lots of lush green vegetation grows in the bottom of dried up sloughs. When this green vegetation floods it dies, takeing the oxygen out of the water, causeing fish to suffocate.It also leaches out tannic acid, which causes the black color, much easier than from fallen leaves.