Yes, RESERVOIR is a Scrabble word!

Letter values in Scrabble US: R 1, E 1, S 1, E 1, R 1, V 4, O 1, I 1, R 1.

Is it a valid word?

RESERVOIR is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a large or extra supply of something
lake used to store water for community use
tank used for collecting and storing a liquid (as water or oil)
anything (a person or animal or plant or substance) in which an infectious agent normally lives and multiplies

Also: artificial lake, man-made lake.