Lack of wind and solar can happen but at scale there is plenty of hydro energy stored to cover significant edge cases. Rivers can handle significantly more flow than current hydro releases without causing flooding, we just need to retro fit existing dams and then reduce their generation most of the time to reserve that capacity for when it’s needed. Aka averaging 6.6% over 365 days a year ~= 80% for 30 days.
Also, be careful when looking at wind and solar minimal percentages. It’s the difference between median output and minimum output that matters not maximum output. Long term it’s likely something like 30 to 50% of all solar generation is going to be wasted simply because it’s just that cheap.
Dams don’t release CO2, they can produce trivial amounts of methane but that breaks down fairly rapidly in the atmosphere. It’s only when vast amounts of methane are released at the same time or the source is continuous that methane is an issue.
In terms of fish, large dams are needed for flood control and water. But rivers and streams often have huge numbers of small dams that are equally problematic and far less useful.
Also, be careful when looking at wind and solar minimal percentages. It’s the difference between median output and minimum output that matters not maximum output. Long term it’s likely something like 30 to 50% of all solar generation is going to be wasted simply because it’s just that cheap.