https://www.forbes.com/sites/jonbruner/2011/10/20/the-high-s...
Things like "there's more potential available in the spring than in the fall (or winter).
> Those margins are doubly difficult to master given the temperamental pulse of a river whose volume increases by a factor of five every spring as snow melts in the Rockies. Within the river’s seasonal changes come manmade fluctuations: every morning its dams awaken the Columbia with surges of water to satisfy the Northwest’s demand for electricity, and every evening the dams tighten their gates to put the river back to sleep. And every other second, an automated system assesses the supply of electricity against demand and makes tiny adjustments to the volume of water moving through each dam’s turbines.
Things like drought will also change that capacity of generation. https://www.hcn.org/articles/the-21st-centurys-hoover-dam
Regarding base load - https://www.e-education.psu.edu/eme807/node/667 - table 9.1 has the capacity factor.
> In the table above, the lower the capacity factor, the more susceptible the system to potential interruptions or drops in performance. We can see that solar and wind technologies, which are notoriously weather-dependent have the lowest CF numbers. At the same time, nuclear power and coal systems are most advantageous when operated continuously and at full load.
Nuclear has a capacity factor of 90.3. Hydro is 39.8. Concentrating solar is 33, wind is 20-40 range depending on geography and photovoltaic solar is 15-19.
More on that concept - https://en.wikipedia.org/wiki/Capacity_factor
Hydro is the best of the renewable non-fossil fuel base load sources, but it still is poor compared to nuclear and fossil fuel energy generation for base load.
New Zealand has a power shortfall about once every ten years when lake levels drop towards critical levels. In 1992 it was severe enough that nationwide cuts of 15 per cent were needed and the GDP dropped by 0.6%. https://www.nzherald.co.nz/nz/how-we-learned-the-lessons-fro...
Hence, it makes sense to use hydro for handling peaks, not baseline supply.