Richland is the Montana county with the highest median household income at $66,572 compared to the state average of $50,801. Blaine County has the lowest median income $35,506, according to a report on the 24/7 Wall Street website.

Richland County, with just 11,405 people, had the smallest population of any county that ranked as the wealthiest in its state in the report.

Nationwide, the median annual household income is $57,652. It varies widely by state, from $42,000 in Mississippi to nearly $79,000 in Maryland. Despite these differences, each U.S. state — even the poorest — is home to at least one county in which the median annual household income exceeds the national median. In some states, the wealthiest county’s median income is more than double the nation’s median,

Also, in every state other than Connecticut, there is at least one county where the typical household earns less than the national median annual household income of $57,652. In the vast majority of states, there are counties where most households earn less than $40,000 a year.

Montana joins North Dakota, Wyoming and Nevada, in breaking a general rule about wealthiest counties, of having a relatively well-educated population. In 46 states, the bachelor’s degree attainment rate in the wealthiest county is higher than the statewide rate, but in these four states the richest county’s economy is dependent on oil or mineral extraction — industries which are often the source of high-paying jobs that do not require a college degree.

Unlike most of the other wealthiest counties, Richland has a significantly lower college degree attainment rate. Less than 20% of area adults finished college, compared to more than 30% of Montana adults overall. Richland County has a poverty rate of 5.9 percent and its October unemployment rate was 2.9 percent.

Richland County residents have been able to find good paying jobs in part because of their proximity to North Dakota and jobs in the Bakken Shale during the oil boom there. Some 17.9% of county workers are employed in the agriculture, fishing, hunting or mining sector, more than nine times the national share of workers.

By contrast, Blaine County has a poverty rate of 32.7 percent and an unemployment rate of 3.6 percent.

Nearly one in every three residents of Blaine County, live below the poverty line, and a majority area households earn less than $36,000 a year. Poorer areas typically have lower average life expectancies, and in Blaine County, life expectancy at birth is about 76 years, 3 years shy of the national average.

Blaine County is located between the Canadian border and the Missouri River and contains the Fort Belknap Indian Reservation, home to the Gros Ventre and Assiniboine Tribes. About half of the county population identifies as Native American.

Nationally, many of the wealthiest counties are within commuting distance of major cities. In all but seven states, the wealthiest county is located in a major metropolitan area. These major population centers are home to a greater concentration of high-paying jobs, which can help keep unemployment low and incomes high.

To identify the richest county in each state, 24/7 Wall St. reviewed median annual household incomes for each U.S. county using 5-year estimates from the U.S. Census Bureau’s 2017 American Community Survey.

Low-income counties tend to have much in common. They are largely rural and have relatively weak job markets. They also tend to have low average life expectancies and declining populations.

In Wyoming, the wealthiest county is Campbell County with a median household income of $80,178, the state’s median household income is $60,938.

Williams County is the wealthiest county in North Dakota, with a median household income in the county is nearly $90,000, almost $30,000 higher than the state average, and $32,000 higher than the national average.