Well that depends on the dam. For something like a hoover dam. They would dewater deconstruct and rebuild. For something small they may build a coffer dam in front of it, with water going into a secondary run off, and tear it down and rebuild/fix.
I have been watching a lot of videos on YT about this very subject. The amount of no longer economically viable dams we have is quite large. But the company that built the thing probably went out of business decades ago. The local areas saw 'hey free lake' and built tons of houses along the shore, and see the lakes as a benefit to their area (recreation/tourism).
Taking one out is a huge undertaking. As you have to make the 'hey free lake' people happy. You also can not just blast it wide open or you end up creating a flood. The previous lake is now just a big pool of 100+ years of mud. So you have to plan out how to dewater carefully, re-seed (with plants native to the area), and maybe even rebuild the bed of the river/stream. All of that costs decent amounts of time and money.
You could say 'oh the guys who built it should have paid for it to be removed'. They are long gone and the money is spent, there is no company to sue, and the 4th gen heir may live in a single wide. We have what we have and have to go from there.
Usually what happens is if the current dam is stable, nothing is done. If fixing the dam costs less than rebuild, they fix it. If fixing the dam costs more than deconstruction, then it is removed.
> the company that built the thing probably went out of business decades ago.
Aren't these things typically owned by a co-op or public organization? If they are privately owned and producing electricity I can't see how they went out of business. Once the dam is built and operational the damn thing prints money.
In some cases yes. In others they were just used for energy to turn a flywheel for sawing wood and no electricity was involved. For the electric ones they may have been able to generate 200MW at their peek just after they were built. But after a hundred years of sediment they may peak out at 50. If the cost of fixing is more than it would generate in say (pick a number) 10 years they may decom it. Which is its own set of challenges. Most of the documentaries I see start off with 'this dam has been here since 1930 and the company went out of business in the 50s'. As most of the reason for the smaller dams to exist have been superseded by better tech. These things are typically 5-10 ft high. They are not much. But there are thousands of them.
Also many of these things are not 'automatic' they need care. Such as switching things at the right time, replacing parts, etc. You may not even be able to buy the correct switches or motors anymore (the NY subway has this issue) and maybe you can get one used from some other dam that is being decommissioned. So they just turn the thing off and walk away. Usually they cut some deal with the local gov to take control. Who think they are getting a 'free park' when they are getting a giant mess. Sometimes they even say 'you do not want it' and the local gov is dead set on getting it for a park. So you end up with things like love canal.
I have been watching a lot of videos on YT about this very subject. The amount of no longer economically viable dams we have is quite large. But the company that built the thing probably went out of business decades ago. The local areas saw 'hey free lake' and built tons of houses along the shore, and see the lakes as a benefit to their area (recreation/tourism).
Taking one out is a huge undertaking. As you have to make the 'hey free lake' people happy. You also can not just blast it wide open or you end up creating a flood. The previous lake is now just a big pool of 100+ years of mud. So you have to plan out how to dewater carefully, re-seed (with plants native to the area), and maybe even rebuild the bed of the river/stream. All of that costs decent amounts of time and money.
You could say 'oh the guys who built it should have paid for it to be removed'. They are long gone and the money is spent, there is no company to sue, and the 4th gen heir may live in a single wide. We have what we have and have to go from there.
Usually what happens is if the current dam is stable, nothing is done. If fixing the dam costs less than rebuild, they fix it. If fixing the dam costs more than deconstruction, then it is removed.