The root of the OP's question is frequency - I read a book a little while agao, Electrical Timekeeping by Frank Hope-Jones, and in there was pictured a clock used at power stations. Well, it was two clocks, a mechanical clock and an electric clock, and it was the job of the power system operators to make the clocks stay together. This requirements persists to this day; the long term frequency accuracy of any civilised country's electrical grid is astounding.
Other than that, I learned (from this very forum) that there are only two controls for syncronised alternators that matter, one is field strength, and the other is throttle. In the case of a hydro generator, then the throttle is the gates that determine the flow of water to the turbine.
Edited to add - further to what Colin noted above - at the
Dinorwig pumped storage power plant in Wales, they go from offload to nearly 2GW of on-line generation in about 15 seconds. This plant is designed to cope with millions of households putting their electric kettles on at the advert breaks in soap operas. What they do there is spin the turbines up as motors, and then open the floodgates (literally) at just the right moment...