The Salton Sea shouldn't exist at all. Had arrogance not overtaken those thinking they'd bring irrigation to the desert, the Salton Sea would still be the bone-dry Salton Sink. But after the canal and headgate failure, et. al., and the Colorado River flowed into the Salton Sink for two years, now we go from Sink to Sea. (And to be fair, it wasn't all due to arrogance; several floods one after another didn't help.)
In short, the Salton Sea was the result of what was probably one of the worst humanitarian and ecological disasters in California history.
The evidence is that this isn't the first time there was water there. Regardless of what contribution human activity made this time, the area has a long history of periodic flooding creating a temporary lake as evidenced by geological markers and Indigenous oral traditions.
I considered adding "...under current ecological conditions", but I though it obvious that natural flooding cycles were not responsible for this incarnation of the Salton Sea. I guess not. There is a good documentary on the Salton Sea entitled The Miracle of the Salton Sea that has outstanding old film footage from the early days, and explains how the current incarnation came about, though the Wikipedia page probably gives the same, if less well-presented, information.
My previous remarks about agricultural runoff already agree with your observation that this incarnation and its duration is substantially influenced by human activity.
I will note a pet peeve of mine: humans are animals indigenous to this planet. We didn't fly in from another galaxy for the express purpose of fucking with it for funsies.
Other animals shape their environment and alter the landscape. Alligators help create swamps and keep them alive. Beavers build dams. Etc.
Not saying humans shouldn't be mindful of the consequences of their actions, but it's a tired trope that human activity is somehow fundamentally different from that of other animals.
Like all animals, we eat, fuck, and die, but humanity's ability to alter the landscape far exceeds that of alligators and beavers. Our combined and fantastically developed use of reason, tools, and written language have elevated humanity to a fundamentally different level. We're each of us now demigods and the world is our golden goose.
Only humanity and celestial bodies can kill nearly all life on Earth. Only humanity can so dramatically raise atmospheric carbon dioxide PPM as we've done. Only humanity engineers earth and waterworks on hundred-km scales.
Sure, it's our home planet, too and we have a right to be here and alter our environment to support our existence like every other animal does. But no other animal can fuck up the planet more than humans can and do. No other animal even comes close.
Pretty sure bacteria made a much more massive difference to the earth's atmosphere/ climate than humans ever have/will do. Accepted it wasn't a single species.
In short, the Salton Sea was the result of what was probably one of the worst humanitarian and ecological disasters in California history.