When I was young I was told that the dam at Keokuk, IA generated 25Hz
> power. Since construction was completed in 1913, this may well have
> been true. I was also told that the power was by then converted to
> 60Hz and fed into the grid.
This is probably no longer true, but in the 60's/70's, Union Electric (as it was then) still had a 25 Hz grid. Keokuk in particular had a number of electrochemical industries that were on this grid. By the time Griffin Wheel (an electric arc furnace operation) came to town, no new customers were being connected at 25 Hz, and Griffin used 60 Hz power.
I don't know what proportion of Keokuk dam 25 Hz production was converted to 60 Hz.