An above ground water storage device includes a plurality of modular walls coupled together in series to form a substantially continuous perimeter. Each of the plurality of modular walls includes a wall panel that is inclinable with respect to a horizontal orientation, and a support base that is coupled to a portion of the wall panel. A liner covers an internal side of the plurality of modular walls and an area encompassed by the continuous perimeter. At least one modular corner wall is positioned with the plurality of modular wall panels to form a corner in the substantially continuous perimeter. The at least one modular corner wall has an angled side with an angle corresponding to the inclined angle of an adjacent modular wall.