Yukon Flats National Wildlife Refuge
It encompasses most of the Yukon Flats , a vast wetland area centered on the confluence of the Yukon River , Porcupine River , and Chandalar River . 
<triplet> Yukon River <subj> Porcupine River <obj> tributary <subj> Chandalar River <obj> tributary <triplet> Porcupine River <subj> Yukon River <obj> mouth of the watercourse <triplet> Chandalar River <subj> Yukon River <obj> mouth of the watercourse