I am a climate scientist and geologist and think that climate, geology, and geography are incredibly interesting fields that people deserve to know more about. If you have any questions that you’ve sat with for a while, are just curious, want to know more about future or past scenarios, or even have worldbuilding questions, feel free to ask!
It’s quite possible that’s the reason, and we do know that inland waters in rivers and streams are also increasing in temperature. At least in the cases of the Rio de la Plata and Congo, it certainly appears to be the case. As for the Temur, I think there isn’t as easy an argument to be made, as it drains into a pretty isolated section of sea that would naturally be prone to heating more easily. The Congo looks like the strongest case, especially because it has no estuary. That being said, I’m not going to commit to an answer because I don’t have the data myself.