I was driving through the town called Klamath Falls and asked local where are the falls. She said "I've been living here for 30 years, there are no falls". I now wonder if there were no falls anymore due to the dams.

Just moved to klamath falls. 4k feet elevation, in a frozen desert with a massive lake. On the border of California.

This is an odd town in every way. I learned this from an older local today. So grain of salt.

Apparently. Klamath was a huge lumber town, and rivaled San Francisco for who would be the larger city...hahah.

But old boys not only shut down i5 going through, but kept a strangle hold on all activities in town for a century.

The unfunny joke I was told. "Klamath Falls will become a decent town when the last 12 old boys die.".

Grain of salt here. Also the spotted owl was a bunch of Eastern Lumber companies funding hippies to shut down western lumber companies. And if you wore a tye dye shirt in town in the 80's. That was a stabbin.

So much salt there.

The dams have been there for decades, but only recently plummeted and no one can prove why? This doesn't inspire confidence.

The salmon populations has been dropping for a century and have reached critical levels in the last few decades. This is not a recent phenomemon. There are many causes and they are understood quite well: dams block spawning habitat, water temperature increases, food web collapse, overfishing, habitat destruction, and algae blooms from pollution are all extensively documented in the literature. Where do you get the idea that scientists are flummoxed by this?

How much are each of those contributing? They don't know. They can't make any predictions about what effect changes will have. The link between temperature and fish numbers didn't pan out a few years ago here in the PNW when water temperatures were higher but we had a boom year in salmon numbers.

If it's really serious let's get the gill nets out of the rivers and let the numbers rebound. But that's never on the table; just taxpayer's billions for destruction of carbon-free electricity.

Yes and not. Most of it is cultured salmon. Cultured and wild are linked so in part we could say that "there is less wild salmon because people eat more domestic salmon". Is complicated.

They aren't understood well though, are they? I mean they JUST discovered the HUGE impact that tire wear runoff from roads causes to fish:

https://www.science.org/content/article/common-tire-chemical....

Not just us. Both terrestrial and ocean ecosystems depend on salmon.

Is there a word for species-centrism? Ah, there is. [1]

My point being, this is probably more about food for wildlife than humans.

[1] https://en.wikipedia.org/wiki/Anthropocentrism

I'm not knowledgeable in Pacific Salmon, but this is a phenomena seen in lots of species, and has a fairly basic biological explanation. A species can be exposed to individual threats that they can still cope with and adapt to, resulting in little immediate decline. But as more of these threats accumulate, eventually the sum of the whole overwhelms the species, and can lead to a sudden drop.

A steep drop can follow a more gradual decline, as well, due to loss of genetic diversity in the population making the population less able to adapt to another unrelated threat. Check out the Extinction Vortex.

The reason why is systemic pressure, as mentioned below, but the primary determinant right now seems to be raised river temperature which is the result of a number of factors, including regional warming and dams, and also results in increased toxins.

One of the only practical ways to decrease river temperatures is to allow the river to be larger, and to flow — that is accomplished by removing the dams.

Don't you mean allow the river to be smaller (i.e. flow faster and have less width and therefore less exposed surface area)?

I wonder what sort of options there are for shading a river? Floating solar panels are used in some places, and have the advantage that it's relatively easy to rotate a whole floating structure to face the sun rather than having to mount every single panel on a heliostat.

More houseboats would be a possibility, but the number required to make any noticeable difference would effectively turn a river into a city, which could have other negative side effects.

Setting up floating barriers and growing azolla or some other water-covering crop might be an interesting option. Not sure how much that would affect temperature, though; it might even cause more heating as the dark leaves convert more light to heat than open water. Also the water might move too fast or be too turbulent to be able to grow water plants. Or if successful, it might cause more water to be lost by evaporation.

(In a lot of cases, removing dams might be the best option, but some dams like Bonneville or Hoover are producing substantial amounts of energy and it would be pretty hard to do without them.)

If a river is smaller then it thermally approaches the surface soil temperature and is more subject to small changes in topology.

If it is larger more of it has more thermal isolation from not only the sun but also the ground. Since most water comes from elevation or “cool mountain streams” or melt, this is a factor, along with the time to get to the sea, because a larger river can carve and maintain a more direct route. It can also be deeper, so more of it is shaded from the sun by the materials it carries.

Deliberately shading it might be an option but a living river is also dependent on photosynthesis so…