A large oak tree could produce up to 1,000 pounds (450 kg.) acorns every year. According to the authors of The Natural World of the California Indians (1980): "The total annual acorn crop for Central California would have run into the millions of tons, but only a small fraction of the yield was gathered each year by the Indians. The bearing season for oaks is only a few weeks, and Indians were not the only collector of acorns. Squirrels, insects, woodpeckers, and bears competed with the Indians for the acorn crop." In 1844 John C. Frémont saw an Indian village where near to each house was "a crate formed of interlaced branches and grass, in size and shape like a very large hogshead. Each of these contained from six to nine bushels of acorns."
Alfred L. Kroeber, an anthropologist, who spent some time living with Indian tribes in California, has argued in Handbook of the Indians of California (1919): "The cache or granary used by the Miwok for the storage of acorns is an outdoor affair, a yard or so in diameter, a foot or two above the ground, and thatched over, beyond reach of a standing person, after it was filled... The natural branches of a tree sometimes were used in place of posts. There was no true basket construction in the cache; the sides were sticks and brush lined with grass, the whole stuck together and fied where necessary. No door was necessary: the twigs were readily pushed aside almost anywhere, and with a little start acorns rolled out in a stream. Even the squirrels had little difficulty in helping themselves at will."
In September 1849, Lieutenant George Derby, while camped on the bank of the Feather River, wrote: "About 200 yards above the farm house is situated a rancheria of Indians, about 300 in number. They had just commenced the collection for their winter stock of acorns, and had many high baskets, containing probably 40 or 50 bushels of this species of provender lying about."
All acorns produced by oaks contain tannin, which is very bitter. The Native Americans dealt with this problem by removing the acorn hull and to grind the interior into a flour in a stone mortar or on a flat grinding slab. They then constantly poured warm water over the flour to leach out the tannin. The leached flour was then mixed with water in a watertight basket and boiled by dropping hot stones into the gruel. Clover, grass seeds, mushrooms and pond lily roots, would be added to give it more flavour. The cooked mush was then either drunk or eaten with a spoon. Sometimes it was baked into a cake.
In 1855 a federal Indian Agent in California reported that the acorn crop had "failed completely in the last three years". It was claimed that Native Americans were forced through the woods for bark-covered trees that had been used by woodpeckers to store acorns. Some tribes stored acorns on the edge of a stream. This mud immersion method neutralized the bitter tasting tannin.
The total annual acorn crop for Central California would have run into the millions of tons, but only a small fraction of the yield was gathered each year by the Indians. The bearing season for oaks is only a few weeks, and Indians were not the only collector of acorns. Squirrels, insects, woodpeckers, and bears competed with the Indians for the acorn crop.