From the record
“Salt River Hydroelectric Powerplant, on the Salt River and on the border of Idaho and Wyoming, near Etna, Wyoming, was built in 1938.”
Wikipedia article Salt River Hydroelectric Powerplant, CC BY-SA 4.0.
One of 9 National Register listings published here for Lincoln County · listed in 1993 · category: District · built in 1938.
Sources: the Wikipedia article · the case file at the National Archives · the National Register database, National Park Service