[2] Armstead was located in a narrow valley of the Beaverhead River, near the mouth of Horse Prairie Creek.

Armstead is perhaps best remembered as the eastern terminus and headquarters of the Gilmore and Pittsburgh Railroad, which operated a line from there to Salmon, Idaho between 1910 and 1939.

The Lewis and Clark expedition traveled past the future location of Armstead during its westward trek in 1805.

[3] The construction of Clark Canyon Dam in 1961–1964 created a reservoir that completely inundated the former site of Armstead.

When the water level is low enough parts of the old highway, railroad, and even some building foundations are still visible.