Nan-tathren (S. "Willow-vale", "the Land of Willows") was the valley where the river Narog flowed into Sirion. In Treebeard's song (in LolRlI 90) Quenya forms of the name were used: Tasarinan and Nan-tasarion.
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Read SU120.195.243-4; UT 32.34-5.