Mariposa Dam, located in Le Grand, California, is a crucial federal structure owned and operated by the US Army Corps of Engineers. Completed in 1948, the dam primarily serves the purpose of flood risk reduction along Mariposa Creek, with a maximum storage capacity of 21,500 acre-feet and a normal storage of 4,300 acre-feet. Standing at 88 feet tall with a hydraulic height of 83 feet, the earth dam spans 1,330 feet in length and features an uncontrolled spillway that is 260 feet wide. Despite historically performing well in reducing flood risks to downstream communities, Mariposa Dam is classified as having a high hazard potential due to concerns over spillway adequacy and the potential for water overtopping during extreme flood events.

USACE has conducted a risk assessment for Mariposa Dam, identifying the potential for erosion in the spillway channel and downstream flooding in the event of a breach. To mitigate these risks, USACE regularly updates its emergency action plan, conducts exercises with local partners, and performs inspections and monitoring to ensure the dam's structural integrity. Weather forecasts are used to inform operational decisions, and coordinated warning notifications would be issued to downstream communities in most scenarios. However, during rare and extreme events, such as large earthquakes, there may be limited warning for potential downstream flooding. Despite these challenges, USACE is committed to managing risks associated with Mariposa Dam to protect public safety and infrastructure in the area.