Looks like you misremembered or misread the map? "Lake Lahontan was a large endorheic Pleistocene lake of modern northwestern Nevada that extended into northeastern California and southern Oregon." It was mostly in what's now Nevada, and there's a clearer map here: https://en.wikipedia.org/wiki/Lake_Bonneville#/media/File:La...

Perhaps there are American Indian oral histories of this lake? It would have covered the Black Rock desert to a depth of 500 feet.

“Archaeological evidence along the shore indicates the existence of the lake coincided roughly with the first appearance of humans in the region. ”

you don't have to go that far back. A little more than a century ago the California central valley looked very different. Tulare Lake was the largest fresh water lake west of the great lakes, bigger than Tahoe[1]. Of course there's also Hetch Hetchy, which supplies San Francisco's water[2]. Countless other engineering projects have vastly changed the flow of water in the state.

1 - https://en.wikipedia.org/wiki/Tulare_Lake 2 -https://en.wikipedia.org/wiki/Hetch_Hetchy

It's always seemed so bonkers to me that Yosemite National Park was made in 1890 and yet Hetch Hetchy, which was part of the park, was dammed in the early 1920s. I get that it was a different time and all, but come on, you can't go around destroying part of a NP like that.

Consistent sources of water for cities are really important, and people were historically more willing to make pragmatic tradeoffs.