Been researching this one for several years. There is a better source than any mentioned above. "Terrible Trail: Lost Meek Wagon Train of 1845".
Just found a diary of a woman on a branch of the 1845 train that left Ft. Boise and went to CA. Along the way she found Au too. They stopped at Sutter's Mill, mentioned their gold, and were told to not tell anyone, as negotiations were on-going at that time to make CA part of Union, and finding Au in CA would have increased the price and produced a stampede. Did anyway. Kind of wonder if that didn't have at least some effect on the legend.
Anyway, after Sutter's Mill announced in Jan. 1848 (not '49 as often stated), she returned to the site with her new husband, and they removed several thousand $'s worth from the original site: mostly placer deposit. Then moved to OR, bought a nice piece of property, and farmed for 50-60 years. Died in OR. But may have been the first finder of Au in CA.
As for the Meek Train of 1845, discovery of Au probably near the headwaters of the South Fork of the John Day River, north of Wagontire Mountain. Train was out of water at this point. After finding a tiny spring on Wagontire Mountain, the train send scouting parties north to try to find more water, but most of the train near dying. South Fork water was found, and the entire train moved north as fast as they could. Train remained here for several days recovering before continuing. Oregon Trail not well established at the time, and Stephen Meek was sort of trailblazing it. Realizing the south had little water, moved the successive trail north where is runs today.
Ironically, the South Fork site is about 40 mile south of John Day, where millions of Au removed a few years later. But as far as I can verify that wasn't found until the early 1860's, and even then it was a fluke of prospecting when small amounts of Au found while a company on the way to another gold strike stopped for a few hours, and one of the company found some good color.