DerbyBorn expressed precisely :
I was being shown a new hydro plant at our local river a few days ago (From
the outside) The Project Manager was saying that the speed was finely
regulated by the water flow to give the right frequency. (That was as
technical as he could go on the topic).

This meant that in time of high flow it is throttled back considerably.
Isn't it possible to generate DC and convert it electronically to AC?
Wouldn't this mean that higher speed would give more amps?

In the old days, to put a generator (alternator) on line, they would
have to match the speed of the generator to the mains frequency and the
phases. One method used lamps connected between the two sources, to
match them so the lamps went out and stayed out. No difference in volts
between the two, meant that the lamps were out with no flickering,
speed and phases were therefore matched - at which point the generator
to would be connected. Once connected they would stay in sync, output
could then be increased so the unit was exporting power to the grid.

Nowadays, things are very different. It is all done automatically and
smaller generators use inverters feeding units which convert back to AC
with everything matched up.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk