Central Valley in Three Acts

It is customary to describe the Great Valley of California in two parts:  the relatively well-watered Sacramento Valley in the north, and in the south, the drier and bigger (twice as big) San Joaquin Valley, named of course for the once-great rivers which pass through them.

In terms of biogeography, however, of climate and flora and fauna, it makes more sense to look at the valley in terms of three parts rather than two:  Sacramento, San Joaquin, and Delta.

The Delta region occupies roughly the counties of (ironically) Sacramento, San Joaquin, Solano, and eastern Contra Costa.

This is a region of braided waterways and rich, deep alluvial soil, distinguished above all by its peculiar climate.  Imagine a long, hot hallway with a window in the middle in which is sitting a big old clunky air conditioner with a fan that blows a stream of cold air against the opposite wall, so that the middle of the hallway is quite a bit cooler than either end of it.

The Delta is the valley’s air-conditioner.  Every year in spring, when the valley begins to swelter, the air conditioner kicks into high gear for the next few months.  Rising hot air in the valley sucks in cool, foggy air from the Pacific Ocean, most of which is funnelled through low gaps in the coast mountains, chief among which is, of course, San Francisco Bay:  in other words, the focal point of the very same Delta where the two great river systems converge.

Anyone who has tried to maintain a vertical position at Carquinez or Altamont on a summer day is reminded how sincerely nature abhors a vacuum.  

Of course the wind-tunnel effect diminishes as the cold air enters the valley.  The energy of the wind is dissipated as it spreads out, warms up, and dries out.  Still, it manages to create a cooler-than-average belt almost if not quite all the way across the valley.

And it can be no coincidence that this is the very zone of the valley and adjacent Sierran foothills where certain plants more typical of the Coast Ranges have gotten a foothold, e.g. Trifolium grayii, T. barbigerum, T. amplectens?, T. microdon?

San Joaquin Valley has less than 8” of rain a year.  Massive irrigation systems began bringing water from Northern California in 1951.  Today more than 70% of the valley is irrigated.