Not only was [Timberline Lodge]... publicly funded, built, and operated, but the workers were trained to perform skilled crafts that they had never done. Form and function are magnificently combined. What a model this could have been for planners and builders. Train men and women to build structures, roads, parks, trails, lodges, and not only do we get aesthetically pleasing things, but we also gain a skilled workforce. Michael Yates
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(Another great road story. I have visited some of the places and towns you wrote about a long time ago, mostly before they went through transformations you describe---on road trip vacations as a kid.)
There was reason for this aesthetic, which you probably know, but I am not sure the rest of the list does...
It comes from a combination of sources in art and architectural history. The most well known source was the Arts and Crafts movement from England that was taken up here in the US before WWI and adapted to our history and landscapes. There were several important concepts like truth to the materials. Another key concept was looking at the landscape and using the natural materials it provided. Simple examples are building log cabins in richly forrested areas. Or for deserts with the right kind of clay deposits, use adobe, tile, plaster and other materials from a rich mineral environment like the US southwest deserts.
A lot of this art and art history made its way into the WPA system just by hiring people who were trained in these craft traditions. There were also linkages to various social reform and leftist movements seeking to `humanize' the brutality of 19thC industrial development. Here is a wiki that covers it:
http://en.wikipedia.org/wiki/Arts_and_Crafts_Movement#United_States_of_America