A post office called Alder was established in 1902, and remained in operation until 1975.[2] The community was named for the alder trees near the original town site.[3]
According to the United States Census Bureau, the CDP has a total area of 3.24 square miles (8.4 km2), of which, 3.21 square miles (8.3 km2) of it is land and 0.03 square miles (0.08 km2) of it (0.83%) is water.