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The British lexicographer John Stephen Farmer (1854-1916) explained the phrase California widow as follows in Americanisms—Old and New. A Dictionary of Words, Phrases and Colloquialisms peculiar to the United States, British America, the West Indies, &c., &c., their Derivation, Meaning and Application, together with Numerous Anecdotal, Historical, Explanatory, and Folk-Lore Notes (London: Privately printed by Thomas Poulter & Sons, 1889) [page 116, columns 1 & 2]:

California Widow.—A married woman whose husband is away from her for any extended period; a “grass widow” in the least offensive sense of that term. The expression dates from the period of the Californian gold fever *, when so many men went West, leaving their wives and families behind them.

* The California Gold Rush began in 1848.

—Cf. also grass widow and grass widower.—Cf. also golf widow.

These are, in chronological order, the earliest occurrences of the phrase California widow (also Californian widow) that I have found:

1-: From Bell’s Life in Sydney and Sporting Reviewer (Sydney, New South Wales, Australia) of Saturday 9^th February 1850 [page 3, column 3]:

The California Widow.
FOUNDED ON FACTS.
[A TALE BY T. M. T.]
Chapter I.—The Heroine.

Amongst the passengers from Sydney for San Francisco, by the schooner Deborah, was a highly respected tradesman of Lower George-street, who had so far followed in the track of his renowned namesake as to have secured in matrimonial bonds a Bella Fornarina of charms scarcely less superlative than those of the original, whose fascinations shed a halo of romance about the life and history of the great painter.
Gold, the ungallant arch-demon, however, entered the lists with beauty, and came of [sic] the victor. The husband succumbing to the irresistible influence of the tempter, had resolved to seek his chief temple in the depths of California, leaving the lovely partner of his joys and sorrows, but not of his adventures, to brave alone the frowns and smiles of capricious fortune.

2-: From The People’s Advocate and New South Wales Vindicator (Sydney, New South Wales, Australia) of Saturday 23^rd February 1850 [page 4, column 2]:

Contempt of Court.—Tolerated Libel.—In an assault case at the Police Office on Thursday morning before Messrs. Broughton and Brown, two female defendants—“Californian widows”—named Chawner or Conlan and Gardiner were each fined 5s. and costs for assaulting a Mrs. Drumm.

3-: From the Weekly Wisconsin (Milwaukee, Wisconsin, USA) of Wednesday 27^th February 1850 [page 3, column 3]:

Attempt at Extortion—A California widow in bad business.—The Chicago papers state that a Mrs. Herrick, whose husband has been for some time absent in California, was, on Friday, bound over in the sum of $400, to answer to the charge of attempting to extort money from a prominent physician in that city.

4-: From the Buffalo Morning Express (Buffalo, New York, USA) of Saturday 23^rd March 1850 [page 1, column 7]:

That incorrigible wag of a Noah perpetrates the following advice to ‘California widows.’ As there are some in almost every place, we publish it for the benefit of this region:

Advice to Ladies whose Husbands are Digging Gold in California.

As a large number of our fellow-citizens have left their wives behind them, in their search for hidden treasures, we trust that it will not seem disrespectful or impertinent if we offer a few words of advice to those temporarily widowed ladies.
As it is quite enough for one of the family to be enduring hardship and privation, it is incumbent upon the ladies to make themselves as easy and comfortable as circumstances will admit. The most reasonable occupation will be the planning of new dresses, dashing equipages, and fine parties for the future. To save time, it would be as well to look over the dry goods stores, milliners’ and dress-makers’, jewellers’ and furniture ware-houses, so as to know exactly what to purchase when the gold arrives. It may be well, also, to take time by the forelock, and order such sumptuous articles as are not kept on hand. If merchants will trust, so much the better; you can have your things without waiting for tardy remittances.
When the first invoice of the gold comes, your friends will probably tell you about savings’ banks, city stock investments, or stupidity of that sort.—Of course, you won’t think of such a thing. You will now buy your dresses and jewelry. Some would say, pay your debts; but those who trusted you can surely wait for the next remittance. There must be plenty more where this came from.
[…]
[…] While you are flaunting in your new finery, your husband is working in the water, lying on the ground, and eating pork. What does this matter? Put your pleasures against his privations, and there will be still quite a pretty average of enjoyment between you. Besides, riches take to themselves wings. You may never have another chance to see life; and if, one of these days, instead of a remittance of gold, you hear that your husband has died of a fever, or that his luck as a gold digger has failed him, you will have spent some happy moments, and at the worst you can go back to your sewing.

The expression California widow soon gave rise to California widower, designating a married man who has left his wife to prospect for gold in California. The earliest occurrence of the latter expression that I have found is from the San Andreas Independent (San Andreas, California, USA) of Saturday 21^st February 1857 [page 2, column 1]:

What we most need in California, is an addition of virtuous females, who will bring with them the refinement of the older states, and stamp its impress upon society here, and shed upon all classes that benign influence which they alone possess.
[…]
To bring about this state of affairs, let those who are here, having families in the Atlantic States, send for them, settle down, and surround themselves with the comforts of home, even if they intend to remain here but a few years. […]
Times are not now as they were a few years ago, when the miner was constantly moving from place to place, and never had any permanent location. In those days it would have been inconvenient to have moved a family, with the household furniture, at every remove the miner himself made. But now, miners are in the habit of locating themselves, in some particular section, and there remaining.—When the California widower locates himself, is he not satisfied that he would be happier and more comfortable to have his [illegible]