Alder, also known as Otter Reservoir, is a private water resource located in Idaho's Owyhee County. Built in 1909 for irrigation purposes, this earth dam stands at a height of 21.2 feet and has a maximum storage capacity of 960 acre-feet. The reservoir is fed by Otter Creek and Marys Creek, covering a surface area of 120 acres and serving a drainage area of 1.5 square miles.

Despite its historical significance, Alder Dam has been classified as having a low hazard potential but poor condition as of the last inspection in June 2018. The spillway is uncontrolled with a width of 25 feet, and the dam's risk assessment is rated as moderate (3). While the dam meets state regulations and undergoes regular inspections, further risk management measures may be needed to ensure its long-term safety and stability, making it a subject of interest for water resource and climate enthusiasts in the region.