That's Seletar reservoir. You have to enter via the Yishun side (can't remember which avenue, but there's a road running parallel to it, which passes by Orchid Country Club) where there's a road to a spacious parking lot. The view is pretty interesting from there.
Talking about reservoir, do also go to MacRitchie reservoir. There are two trails at the reservoir, one where the runners take, and another one (leading to another direction) where the nature lover go. Quite lovely sights there as well!
Ah! More nature haunt... Bukit Timah Nature Reserve. There's so many trails there (err... actually there's only about 4 or 5) that if you plan nicely, it would be a different experience everytime!