Aluminum might have been available in 1900, but remember that it takes a TON of electricity to refine aluminum. It was probably considered an exotic material and priced accordingly. It was probably not affordable until the major aluminum producing countries (Canada among them) finished the bulk of their hydropower projects ...
A short look at Wikipedia suggests that many of these dams were constructed in the 50's, 60's, and 70's.
http://www.time.com/
"The first step as part of a giant expansion of Canada's aluminum industry was the construction of the world's biggest aluminum plant in Arvida, Quebec, near the new [Shipshaw] dam, in 1941".
- Don Gillies
San Diego, CA, USA