There's also a relatively clear split between rural and nonrural counties. There are 21 rural counties in California (by the U.S. Office of Management and Budget's definition — click here for details), and Sanders leads in 16 of them. And those counties are also heavily clustered in the northern part of the state. His major nonrural victory was in Santa Cruz County, that small (but high-population) brown spot on our map on the west side of the state. And those few eastern counties he also won? They're also rural.
The rural counties trend isn't entirely separate from the white voter trend. California's northern, rural counties also tend to be whiter than the rest of the state. (Likewise, the rural population nationwide is whiter than the nonrural population.)
By the way, Kurtzleben is apparently using the Office of Management and Budget definition of rural (see Map 3 and the definitions pages): "All counties outside metropolitan areas in 2003 (based on 2000 census data)." She also observes:
Clinton leads in Imperial County — that southeastern-most one, in the lower tip of the state — by 35 points. That county, with its 82 percent Hispanic population, is the most heavily Hispanic county in the state, according to a Los Angeles Times analysis. (Because some mail-in ballots still have to be counted, the candidates' leads in different counties could still change.)
Of course, the state also has significant black and Asian-American populations. It appears that in some counties, minority voters helped push Clinton's vote total up, while white voters bolstered Sanders' totals in many places. Indeed, the five counties where Clinton got her lowest share of the vote are more than two-thirds non-Hispanic white.