E. Beckwith #1, also known as Lost Lake Slough, is a privately owned irrigation dam located in Gunnison, Colorado. Constructed in 1935, this earth dam stands at a height of 20 feet with a hydraulic height of 41 feet and a structural height of 43 feet. It serves primarily for irrigation purposes, with a storage capacity of 864 acre-feet and a normal storage level of 360 acre-feet. The dam is situated on Anthracite Creek-Tr, with a drainage area of 1 square mile and a maximum discharge of 600 cubic feet per second.

Despite its low hazard potential, the condition assessment of E. Beckwith #1 is rated as fair, with the last inspection conducted in July 2015. The dam has no spillway and is equipped with slide and uncontrolled outlet gates. The risk assessment for this structure is classified as high (2), indicating the need for ongoing monitoring and potential risk management measures. As a key element in the local water resource infrastructure, E. Beckwith #1 represents a critical component in the management of water resources in the region.