Upon completion, Geist Reservoir was the second-largest man-made lake in Indiana, providing approximately 6,900,000,000 US gallons (2.6×1010 L; 5.7×109 imp gal) of water.

Planning for the reservoir began as early as 1913, when hydraulic engineers estimated that White River and Fall Creek would not provide enough water for the increasing needs of Indianapolis.

Geist gradually bought some 5,000 acres (20 km2) in the Fall Creek Valley in the 1920s and 1930s, including the small town of Germantown, which today lies at the bottom of the reservoir.

Situated near the southern shore of the north end of Geist, the new reservoir covers 88 acres (36 ha) and is 220 feet (67 m) deep.

It holds three billion US gallons (1.1×1010 L; 2.5×109 imp gal), equal to almost half the capacity of Geist Reservoir itself.

When needed, up to thirty million US gallons (110,000,000 L; 25,000,000 imp gal) a day can be pumped from Citizens back into Geist.