I had a similar situation in a game I started a couple of months back (but then quit), where I founded several towns that appeared to border rivers, according to every indication that I could find (e.g. graphics, increased commerce on the tile) but Aqueducts nonetheless appeared in those towns' build-lists, i.e. the game didn't recognise that the town was on a freshwater source.
The only thing I can think of is that this is actually some kind of glitch which occurs due to the closely placed river(s)? That is, even though the graphics show two 'parallel' rivers, the branch along the NW of the tile (SE of your Workers) is spurious.
In the Editor, rivers appear to be defined by the tile-corners they connect, rather than the tile-edges they follow. So you can see that kind of thing happening when you try and place rivers manually: you only have limited control over where the river-graphic is placed.
For example, this intended arrangement:
¯ ¯|_|¯|_ _
...might not actually be 'understood' properly: the 'open' tile-edges might end up getting closed off, so you end up with 2 adjacent 'river-islands'.