Munchkin River
The Munchkin River is the river down which the A-B-Sea Serpent and Rattlesnake travel back to the Mer City.
First named in The Royal Book of Oz, ch. 10
and that very morning they had started back to the Munchkin River, and all the celebrities of Oz had gotten up to see them off.
Where this is
placed In Oz worked out
read out of the chapters cited on this page, not stated in any one sentence of them
Royal Book of Oz ch2 establishes the river crossing as being 'in the Munchkin Country of the Land of Oz.'