'Valley' is the usual translation of nan, but it actually has a more precise sense, referring (especially in this context) to a wide, shallow river-valley (as opposed to an imlad, a steep-sided narrow valley). The nan in the name Tasarinan (and its other names Nan-tasarion and Nan-tathren) therefore refers to the relatively flat region or land around the confluence of Narog and Sirion. For that reason, the usual English formulation is 'Land of Willows', though 'land' does not quite capture the more specific implications of the Elvish word nan.