Before Europeans arrived, the Santa Rosa Plain was home to the Bitakomtara, a populous Pomo people who closely guarded their land and gathered for ceremonies on Santa Rosa Creek near today's Spring Lake. After the Spanish came, the Pomo were devastated by violence, land seizures, slavery and smallpox, and their descendants still live in the region.
More about Santa Rosa
Mexican colonists founded Santa Rosa in 1833 and named it for St. Rose of Lima. By the 1820s settlers from Sonoma were already raising livestock in the area, and they slaughtered animals where Santa Rosa Creek meets Matanzas Creek, which is thought to be the origin of the name Matanzas, "slaughter." In the 1830s the family of Maria Lopez de Carrillo, in-laws of Mariano Guadalupe Vallejo, built an adobe on their Rancho Cabeza de Santa Rosa, the first known permanent European settlement, just east of the future downtown.
After the American takeover, a Wells Fargo post and general store opened downtown in the 1850s, and in the mid-1850s Julio Carrillo and other leading residents laid out the street grid around a central square, now Old Courthouse Square. The county recognized Santa Rosa as an incorporated city in 1867 and the state confirmed it in 1868, the third in Sonoma County after Petaluma and Healdsburg. Santa Rosa trailed Petaluma at first, but by 1870 it was the eighth-largest city in California and seat of one of the most populous counties.
Sources
Wikipedia: Santa Rosa, California