Prairie Lake, a small reservoir in St. Charles County with a 1.25 km² watershed, is experiencing leakage through the permeable, chert-rich residuum derived from Burlington - Keokuk Limestone that constitutes its bottom. The present elevation of the water surface (165 m) is 7.3 m lower than the lake elevation in 1982, and the lake area has been reduced from 10 ha to 4.9 ha. Results from a precise topographic and bathymetric survey indicate that the volume of the lake has undergone an 81% reduction, from approximately 576,000 m³ (466 acre-feet) to the present value of 111,000 m³ (90 acre-feet). The principal outflow of the lake is now directed through the subsurface to Weldon Spring, a karst spring located 500 m to the northeast that is 2.3 m lower than the present lake surface. This small karst spring is consistently enriched in d¹⁸O (-4.30 ± 1.5 ‰) compared to other springs in the area (e.g. -7.55 ± 0.7 ‰ for Rockwoods Spring), indicating an evaporated component that has been traced to the lake (Env. For., 2, 99-101). Regional potentiometric maps, diatom tracing and geochemical calculations provide conclusive evidence of this connection. Mixing calculations indicate that Weldon Spring is now a combination of lake and groundwater in an approximately 4:1 ratio. Since construction of the lake, the discharge of Weldon Spring has increased from 0.04 cfs to about 0.5 cfs, suggesting an enlargement of the subsurface conduits. Water balance calculations show that these conduits now drain practically all the recharge to the lake, and their further enlargement will result in the continued lowering of the lake level.