Lake Quad is a large lake, located about two miles south of the Emerald City.
Davy Jones made his way there on his quest to get to the Emerald City, and after his visit to the city he made his permanent residence there, where he could work as a courier for the Wizard of Oz as a courier rather than return to the Nonestic Ocean. (Lucky Bucky in Oz)
The following year, Cap'n Bill made a boat for Ozma out of wood so that she could sail in the lake. He planned to present it to her upon her return from the Forest of Burzee, but Mimics interrupted this plan when they replaced Dorothy and the Wizard. (The Magical Mimics in Oz)