According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. Indian reservations comprise 63.4 percent of the county's land area: The Navajo Nation takes up 60.45% and the Ute Mountain Ute Tribe Reservation another 2.93%. The physical features include three rivers: the San Juan, Animas, and La Plata rivers; also, the Chuska Mountains and Shiprock Pinnacle to the west, volcanic structures, buttes, mesas, badlands, and fertile river valleys.
At the 2000 census there were 113,801 people, 37,711 households, and 28,924 families living in the county. The population density was 21 people per square mile. There were 43,221 housing units at an average density of 8 per square mile. The racial makeup of the county was 52.83% White, 0.44% Black or African American, 36.88% Native American, 0.27% Asian, 0.05% Pacific Islander, 6.77% from other races, and 2.78% from two or more races. 14.99% of the population were Hispanic or Latino of any race. There were 37,712 households, 42.00% had children under the age of 18 living with them, 55.70% were married couples living together, 14.70% had a female head of household with no husband present, and 23.30% were non-families. 19.30% of households were one person and 6.40% were one person aged 65 or older. The average household size was 2.99 and the average family size was 3.43. The age distribution was 32.60% under the age of 18, 10.00% from 18 to 24, 28.10% from 25 to 44, 20.20% from 45 to 64, and 9.10% 65 or older. The median age was 31 years. For every 100 females there were 98.30 males. For every 100 females age 18 and over, there were 94.70 males. The median household income was $33,762 and the median family income was $37,382. Males had a median income of $35,066 versus $21,299 for females. The per capita income for the county was $14,282. About 18.00% of families and 21.50% of the population were below the poverty line, including 26.60% of those under age 18 and 18.20% of those age 65 or over.
2010 census
As of the 2010 census, there were 130,044 people, 44,404 households, and 32,457 families living in the county. The population density was. There were 49,341 housing units at an average density of. The racial makeup of the county was 51.6% white, 36.6% American Indian, 0.6% black or African American, 0.4% Asian, 0.1% Pacific islander, 7.3% from other races, and 3.5% from two or more races. Those of Hispanic or Latino origin made up 19.1% of the population. The largest ancestry groups were:
34.8% Navajo
15.0% English
10.7% Mexican
9.1% German
5.9% Irish
4.4% Spanish
2.8% American
1.8% French
1.4% Italian
1.2% Scottish
1.1% Scotch-Irish
1.1% Swedish
Of the 44,404 households, 40.5% had children under the age of 18 living with them, 49.7% were married couples living together, 15.6% had a female householder with no husband present, 26.9% were non-families, and 21.9% of households were made up of individuals. The average household size was 2.89 and the average family size was 3.38. The median age was 33.0 years. The median household income was $46,189 and the median family income was $53,540. Males had a median income of $44,984 versus $30,245 for females. The per capita income for the county was $20,725. About 15.9% of families and 20.8% of the population were below the poverty line, including 28.0% of those under age 18 and 19.1% of those age 65 or over.
