Posy No. 1 is a privately owned earth dam located in Sussex, Wyoming, along Posy Creek. Built in 1995 by designer John Lambert, this dam serves primarily for irrigation purposes, with additional benefits for fire protection, stock, and small fish pond use. Standing at a height of 51 feet, with a hydraulic height of 47 feet, Posy No. 1 has a storage capacity of 2162 acre-feet, making it a crucial water resource for the surrounding area.

Despite its age, Posy No. 1 is deemed to have a low hazard potential and fair condition, as assessed during its last inspection in July 2020. The dam features an uncontrolled spillway with a width of 200 feet and an outlet gate of the slide type. With a drainage area of 15.6 square miles and a maximum discharge of 6650 cubic feet per second, this structure plays a vital role in managing water flow and storage in the region, contributing to water resource sustainability and climate resilience.

Located in Congressional District 00 of Wyoming, Posy No. 1 falls under state jurisdiction and regulation, with permitting, inspection, and enforcement carried out by the State Engineer's Office. With its moderate risk rating and the potential for future modifications or risk management measures, this dam remains a focal point for water resource and climate enthusiasts interested in the intersection of infrastructure, water management, and environmental stewardship.