Kaliningrad
The city is situated on the Pregolya River , at the head of the Vistula Lagoon on the Baltic Sea , with a population of 489 , 359 residents , up to 800 , 000 residents in the urban agglomeration . 
<triplet> Pregolya River <subj> Vistula Lagoon <obj> mouth of the watercourse <triplet> Vistula Lagoon <subj> Pregolya River <obj> tributary <subj> Baltic Sea <obj> part of