Is it really a river square ? I have seen many cases where a tile shows up as a river but the mouse over says it's something else (OK this is also a bug)

According to the city screen the yield was a sweet 3/3/3.

I think one problem is that there aren't any graphics for indicating that the city's town centre is built over a river.  Secondly, whenever I've looked at the map it hasn't shown tile yields in rivers, meaning that as far as the player is concerned, they still can't build on rivers, only capture AI cities built on them, unless the map - being a convenience for the player - is lying and I can build on rivers if I can find a spot like that one.

Actually, that's not entirely true.  I loaded up my beginning-of-game save in cheat mode, revealed the map.  Lo and behold, it's a river tile.

It's Schrödinger's Tile.