Geist Reservoir is a reservoir in the northeastern part of metropolitan Indianapolis, Indiana, USA. It was constructed in 1943 by damming Fall Creek to provide water for Indianapolis. Upon completion, Geist Reservoir was the second-largest man-made lake in Indiana, providing approximately 7,000,000,000 US gallons (2.6×10¹⁰ l; 5.8×10⁹ imp gal) of water.