This image shows many parts of the Hoover Dam complex, including the face of the dam, the generator buildings, the outflow passages and other components.

The caption states:
This spectacular view of Boulder Dam was taken from near river level downstream from the huge dam. Here flanked by towering canyon walls, the dam is seen in the distance. Boulder Dam, – the highest in the world – was built and is operate by the Bureau of Reclamation of the Department of the Interior. Rising 727 feet above bedrock the dam seems actually to be dwarfed by the surrounding canyon and mountains. IN the immediate foreground the Colorado River is seen as it continues its way below Boulder Dam, and on each side of the channel may be seen the “lower portals” of the diversion tunnels which carried the water around the damsite during construction.

–The dam was widely but informally known as Hoover Dam because it was funded and started during the Hoover administration. However, it was named Boulder Dam when completed during the Roosevelt administration, and was only formally renamed Hoover Dam in 1947.

Given the naming used here, it is likely this card dates from the 1940s.