Mio Hydroelectric Plant
The Mio Hydroelctric Plant was the fourth of six hydros built on the lower Au Sable River between 1909 and 1924 by Consumers Power Company, later Consumers Energy. Completed in 1916, it was the first hydro to pass excess river flow through concrete passageways or conduits built into the foundation of the powerhouse rather than over the dam through an above ground gated spillway, which was more costly. Known as a conduit spillway, it was invented and patented by William W. Tefft, a civil and hydraulic engineer for Consumers Power. (A gated spillway was also built.) In 1988 an aboveground concrete emergency spillway was built on the west side of the powerhouse to meet then current federal dam safety standards.