The Willow meads of Tasarinan are mentioned by Treebeard as he carried them to Wellinghall.
In the willow-meads of Tasarinan I walked in the spring
Ah the sight and smell of the Spring in Nan-tasarion.
This, of course, was the spring of his life, so was before the flood.
It was located at the confluence of the Narog and the Sirion, and so it was submerged by the flood.
In the Silmarillion, it's called Nan- tathren, and is mentioned as the most southerly place that the orcs would venture after the fifth battle, and also as the place where the remnant of the people of Gondolin stopped after the fall of Gondolin to mourn the fallen.