Around 5000 years ago a dry period hit mot of (Western?) North America. In this period we see drastic changes in resource usage throughout California, most notably along the coast of Southern California, the Central Valley, and the Bay Area.
In the Bay Area for example, we see the development of lots of mortars and pestles, implying sedentary lifestyles and far more intensive grinding of seeds and nuts into flour than previously, where
manos were dominant instead.
In the Bay Delta/Central Valley, there's still mostly manos being used, but trash middens show that less wetlands fauna and flora were used, presumably because the wetlands were growing scarcer with lass snowmelt from the Sierra Nevada.
In Southern California there's more dependence on ocean resources, and this might be an early period of the really advanced seafaring and canoe building technologies of the area around the Chumash coasts.
IOTL this period led to further intensification and cultivation of wild resources, a sort of sedentary hunter-gatherer region that had a rather amazing population density considering the lack of proper agriculture, but nonetheless did not till the land (whether this is a good or bad thing is up for debate, I'm simply asking what if it hadn't been the case).
My question is what if this period prompted the development of actual agriculture in the region?
As for founder crops, I think
wapato,
goosefoot, or
powell's amaranth would work. The former would likely be a result of Valley people deciding to cultivate the tubers in the receding wetlands to "make up" for less wetlands area in total. The latter two could feasibly be cultivated any of the groups, but considering the Bay Area people began using mortars and pestles around this time for
acorn processing, and with oak trees being rather difficult to plant and harvest in a reasonable time-frame, maybe they would develop a goosefoot or amaranth pseudo-grain agriculture.
Amaranth, being a C4 photosynthesizer, flourishes in hot and dry areas where other plants don't due to incredibly rapid growth and deep roots, but the wild versions are (as with most plants) inconvenient to grow. Yet as a founder crop it may work, and later on other native grasses (grass seeds were used extensively throughout California's prehistory) could be cultivated as well.
There are tons of useful plants that could be cultivated, legumes, berries, tubers, cereals and pseudocereals, etc. But they'd require a culture that understands the concept of plant cultivation already.
I chose 3000 BCE because it's already a period of rapid and drastic change in the region, but theoretically any later drought period might provide the same driving force.
As for consequences, I imagine another heart of agriculture would obviously develop states later on down the road, but would more importantly end up developing continental trade routes, which were extensive IOTL, into something that could more easily spread crops and technologies around.
Maybe maize agriculture would make it to California, and might make it to the Mississippi far earlier than OTL (which was somewhat earlier than 1000 CE). Overall a more connected Americas is a good thing, I'd imagine.