[3] The source of the river's longest branch is near Nine Maidens, about 2 miles to the north of St Columb,50°26′49″N 4°56′35″W / 50.447°N 4.943°W / 50.447; -4.943, at a height of approximately 490 feet above mean sea level.

Tributaries to the river include one that rises between the village of Tregonetha and Castle an Dinas, and one whose source is south-east of Tregatillian.

A smaller river, from Talskiddy, joins the Menalhyl a short distance from its mouth.

Its valley, from St Columb downstream, is called the Vale of Lanherne; it is wooded for much of its length and is popular with walkers.

[1] The bedrock of the catchment area largely consists of Devonian calcareous slates, grits and limestone.