Yes, THALWEGS is a Scrabble word!

Letter values in Scrabble US: T 1, H 4, A 1, L 1, W 4, E 1, G 2, S 1.

Is it a valid word?

THALWEGS is a valid word in the official Scrabble dictionary (US) and the UK Scrabble Dictionary, but not in Words with Friends (ENABLE).

the middle of the chief navigable channel of a waterway that forms the boundary line between states
a line following the lowest points of a valley