Why isn't it 'Ahhoz a folyohoz megyunk, amelyikhez ez a buszhez megy'?
Nothing is going "to the bus," so "bus" shouldn't get the -hez ending.
Oh, köszönöm! Terribly obvious, now that you mention it.
Eventually I'll get my head around the case system. I hope.
ahova ez a busz megy?
No, you are referring to a specific noun X = folyó, so you use the 'that X ... to which' construction (amelyikhez), not the 'where' construction (ahova).