The gap between the wealthiest people in Sonoma County and everyone else is not only vast but also has widened significantly over the past quarter-century, according to a new report.

The top 1 percent of households in Sonoma County saw their average annual income rise 40.8 percent between 1989 and 2013, while the remaining 99 percent of households saw their average incomes decline 10.4 percent during the same period, according to the report by the California Budget and Policy Center, a Sacramento think tank.

“It’s still a pretty dramatic increase in income for the top 1 percent, while you have decreases in the incomes of everyone else,” said Chris Hoene, executive director of the center, which seeks to improve the economic and social well-being of low- and middle-income Californians.

In Sonoma County, the top 1 percent earned an average income of $913,522, almost 20 times more than the average income of everyone else, or $45,794. The gap was the 13th largest among the 33 California regions examined in the report.

Income disparity has become a significant issue in the Democratic presidential debates, where Sen. Bernie Sanders has gained momentum by attacking the wage gap and accumulation of wealth by the richest Americans.

Nearly 17 percent of the county’s total income was funneled to the top 1 percent of households in 2013, up from 11.4 percent of the county’s total income in 1989, according to the center’s report.

Hoene said such trends in widening income gaps can become an obstacle to future economic growth. He said economic growth that is more broadly shared, where people across all income levels experience gains to some degree, are a better harbinger of future growth.

Sonoma County, however, saw average incomes decline for the bottom 99 percent of households.

“You want to at least see the incomes for the other 99 percent keep up with the cost of living, particularly housing costs,” Hoene said. “When you look at Santa Rosa and Petaluma and you see the other 99 percent with declining incomes, you know the answer then is that incomes are not keeping up with the cost of living.”

Closer to the Bay Area’s technology hub of San Francisco and Silicon Valley, incomes actually increased across the board, though probably not enough to keep up with the skyrocketing cost of living in those areas.

In Silicon Valley, the average incomes for the top 1 percent soared 249 percent since the dawn of the Internet, while average incomes for the bottom 99 percent in that region grew only 23 percent, the report said.

The San Francisco region, which includes Redwood City and San Mateo, saw incomes for the top 1 percent jump 219 percent, while incomes for the bottom 99 percent grew only 35 percent. In the San Francisco area, households in the top 1 percent earned an average income of $3.6 million, or 44 times the average income of the bottom 99 percent, $81,094, the report said.

In Napa, the top 1 percent of households enjoyed a 106 percent increase in incomes from 1989 to 2013, while the bottom 99 percent saw only a 6 percent bump.

“The region south of you is experiencing substantial economic gains over a long period of time. Income is in a smaller number of hands,” Hoene said.

In the North Coast region, which includes Lake, Mendocino, Humboldt and Del Norte counties, the widening income gap has largely been driven by a significant decline in the average income of the bottom 99 percent.

In this region, the top 1 percent saw their average paychecks grow only 0.4 percent between 1989 and 2013. But average incomes for the bottom 99 percent shrank by 25 percent during that same period.

The North Coast’s anemic growth in wages for both top- and bottom- income earners reflects a region that continues to struggle economically, Hoene said.

“It’s a pretty minor growth in the incomes of the top 1 percent,” he said.

You can reach Staff Writer Martin Espinoza at 521-5213 or martin.espinoza@pressdemocrat.com. On Twitter @renofish.