There's always going to be some residual power generation. If there's a small town-scale Grid which is powered exclusively by hydro, there's a good chance it could run for a decently long time, but they need some way to offset a relatively constant energy production into the peak hours.

Hydro has some of the same problems above - it doesn't respond to load demand, and if it's on a hybrid Grid, it'll have the same Solar problem.

It's actually a big problem in our energy industry right now: The Duck Curve. https://www.cnet.com/home/energy-and-utilities/the-duck-curve-the-cute-sounding-energy-problem-well-need-to-fix/

We've deployed so much Solar that the Peak energy generation needed (outside solar) is about 2x the minimum (which happens during midday when the sun is highest). That's a huge swing in non-Solar energy demand.

Even without the Duck Curve problem, your night time demand is going to be about 30% less than your day time demand.

So if your Hydro is scaled to handle Peak demand, you would have to shed all that night time power somewhere, or have an Operator close the sluices which feed the generator turbines a bit, to scale back the energy.

If your Hydro is scaled to handle Off-peak/Night demand, you would end up with Brownouts and possibly Blackouts during the day as Grid demand far out-strips generation and you end up back like Texas in 2021.

Edit: Basically the first person to solve and patent an efficient and inexpensive way at Grid scale to store excess Solar from the day to use at night is going to become the richest person in the world ever.