From Tolkien Gateway

Glithui was a small river that rose beneath Amon Darthir in the Mountains of Shadow, beneath a steep and treacherous pass out of Dor-lómin into West Beleriand. It flowed southward away from the mountains for less than fifty miles, after which its waters ran into the Teiglin (north of that river's confluence with the Malduin), and on into the mighty river Sirion.[1]

Voronwë led Tuor across the Glithui as they journeyed east along the Ered Wethrin towards Gondolin.[2] Later, when Túrin escaped from Dor-lómin he used the pass that came down to the Glithui.[3]

The Glithui is shown on the map that accompanies The Silmarillion,[4] but is not labelled there.[5]