> A lot of the dam removal pressure is activist not scientific - people who have come to believe dams are evil because they aren’t natural

Similarly, a lot of dam preservation pressure is reflexive reactionary thinking that if someone wants to remove the dams, it must be because they're a hippie environmentalist and the dam must be saved to show our commitment to Progress.

Dams have a finite lifespan. Rivers carry sediment. The dams slow the flow of water and the sediment is dropped. This fills up the reservoir behind the dam, eventually making the dam ineffective. In addition, ordinary mechanical stresses wear out dams and they're components, so there is a maintenance cost to just keeping them running.

Many failure modes for dams are catastrophic: a release of water and silt all at once into downstream areas.

Worse, many of the dams that were built in the dam-building boom in the US West from circa 1930 to 1965 or so were not particularly well-thought-out, especially smaller privately planned dams.

In the mid-century American Bureau of Reclamation, building dams was like building new chat services is at Google today. While dams, as a concept, are completely critical to making the western united states survivable with mid-20th century technology, many of the actual dams were not good designs, they are the result of a generation or two of engineers responding to promotion incentives within a large bureaucracy, and they should no more be given the benefit of the doubt as good engineering projects than the last abandoned open source project you saw from Google, Facebook, Uber, etc.

Consider Matilija Dam (https://en.wikipedia.org/wiki/Matilija_Dam):

* "In 1941 the U.S. Army Corps of Engineers warned that the dam would not be economically effective, as the steep, erosive topography upstream would cause it to silt up quickly. However, the project moved forward and in 1945 the county issued $682,000 in revenue bonds to fund it. Construction began on 18 June 1946 and was completed on 14 March 1948 at a cost of nearly $4 million, six times the original estimate"
* "Almost immediately after construction, the dam began silting up.[7] The dam traps about 30% of the total sediment in the Ventura River system, depriving ocean beaches of replenishing sediment.[6] Initially, engineers had estimated it would take 39 years for the reservoir to fill with silt,[1] but within a few years it was clear that the siltation rate was much faster than anticipated. In 1964 a safety study was commissioned from Bechtel Corporation, which determined the dam was unsafe and recommended removal."
* The dam was notched twice, reducing its capacity and function, and the reservoir was useless by 2020.
* Ventura county started trying to remove the dam in 1998 (who knows what happened between 1964 and then), but the dam is still there.

Even the good dams don't last forever, and there is no plan to deal with the sediment build up in the West's dams.

However, the Reclamation and Army Corps of Engineers dams are the good ones. The real corkers are the private dams. Consider Rindge Dam (https://en.wikipedia.org/wiki/Rindge_Dam) in Malibu, California, which was privately built by the Rindge family when they controlled the entire Mexican land grant rancho that is present-day Malibu:

* Built 1926
* Completely silted up by 1950, 24 years later. 24 years of "useful" life tops ("useful" is suspect because most Rindge family building projects were weird compliance dodges to preserve control of the ranch. They spent decades building and tearing down a railroad because the law on the books at the time prevented the state from using eminent domain to seize their land for road-building if there was a railroad under construction there)
* Congress authorized removal study in 1992.
* In 2014, dam considered so dangerous due to lack of repairs that the area, which is now in a state park, was closed to the public in 2014.
* The dam can't just be knocked down, what would happen to the 600k cubic meters of sediment that are now trapped behind the dam, that should have flowed down the river for the last 100 years? The plan is to _truck the sediment out_. Some will be dumped in the ocean, the rest in _landfills_.
* The currently scheduled goal to complete the removal project is _2033_. The dam was been functionally useless for its original purpose since 1950. It's 83 year "useless/dangerous" lifespan will surpass it's 24 year "useful" lifespan by 3.5x! Surely _some_ of that is government beauracracy but not all: it's very difficult to unbuild a silted up dam. It's harder to undo things than it is to do them.

I think there is much more significant "religious faith" in the sanctity of dams than there is "belief that dams are evil because they aren't natural" in the United States. Dams are a powerful symbol of America's mid-century confidence in it's ability to bend nature to its will. Hoover Dam is more than a tourist site, it's something closer to a civic-religious site, like the Lincoln Monument. So is Glen Canyon. Grand Coulee Dam is known to a lot of people as "The Dam That Won World War II" for it's role in powering the aluminum-smelting plants and nuclear material refinement sites in the Northwest. How many pieces of infrastructure are considered war heros in the US?

The sanctity of dams is way more obvious in the northeast. There's hundreds and hundreds of abandoned dams on every trickle of water in the mid-Atlantic and New England, all to power mills that stopped milling 100 years ago, but the dams are still there, and the fish are not.