St. Marys Lake, also known as Geo. R. Stewart Reservoir, is a privately owned water resource located in Idaho Springs, Colorado. This reservoir was completed in 1889 and primarily serves as a water supply source for various purposes including fire protection, stock, irrigation, and small fish pond maintenance. With a storage capacity of 51 acre-feet, St. Marys Lake covers a surface area of 7 acres and has a maximum discharge rate of 220 cubic feet per second.

Despite its historical significance and critical role in water management, St. Marys Lake poses a significant hazard potential with an unsatisfactory condition assessment. The dam, standing at a height of 9 feet and a length of 210 feet, has an uncontrolled spillway type with a width of 40 feet. The risk assessment for this reservoir is classified as moderate, highlighting the need for ongoing risk management measures and regular inspections to ensure public safety and environmental protection. As a key component of the water infrastructure in Clear Creek County, St. Marys Lake requires careful monitoring and maintenance to mitigate potential risks and ensure sustainable water resource management for the future.