A cat in a river was drowning in a River when... Is a bird? Is a plane? ... NO it's Super Dog!
It's quite unique and precious to see a dog risk his own life for a cat because they are traditionally at odds.
Some may argue that a cat can swim but so can I. The fact is a river can have odd currents as well as cold water temperatures that will put a body into shock. Even the best swimmer can run out of energy trying to fight currents or panic which can stop rational thinking.
I've actually sen my Son slip into a cold mountain river on a very hot day (99F - 37C), and he totally freaked out when he couldn't get back up the slimy rock. He's on the swim team but the shock of the cold water somehow made him irrational. I immediate ran to the shore and started quickly walking to him in the water as I was yelling at him to turn to swim towards me. He didn't and started crying crazily.
When I got to him after about 7 seconds, he was starting to going under the water a few times, and I think he would have drowned - NO JOKE! The water was shallow enough for me to stand and he grabbed my neck sobbing. After he calmed down, I asked him why didn't listen to what I said, and he said all he could think of was get out of the water. And he was trying to the way he slid in and that is all that was in his mind. He didn't hear me he said.
You hear of freak drownings, and it can literally happen like that under certain conditions. And it's the same for all warm blooded mammals.