The Moreau River flows east-northeasterly through the upper central parts of Dewey County, discharging into the Missouri River near the county's NE corner. Smaller drainages move runoff water northward from the central-eastern portions to the Missouri River, discharging near the community of Promise. A significant arm of the Missouri River forms the county's southeastern border. The county terrain consists of rolling hills, sloping southeastward and dropping off into the Missouri River basin.[5]
The county has a total area of 2,445 square miles (6,330 km2), of which 2,302 square miles (5,960 km2) is land and 143 square miles (370 km2) (5.8%) is water.[6]
The eastern portion of South Dakota's counties (48 of 66) observe Central Time; the western counties (18 of 66) observe Mountain Time. Dewey County is the easternmost of the SD counties to observe Mountain Time.[7]
U.S. Decennial Census[9] 1790-1960[10] 1900-1990[11] 1990-2000[12] 2010-2020[1]
2020 census
As of the 2020 census, there were 5,239 people, 1,705 households, and 1,180 families residing in the county.[13] The population density was 2.3 inhabitants per square mile (0.89/km2). There were 1,923 housing units.
2010 census
As of the 2010 census, there were 5,301 people, 1,730 households, and 1,239 families in the county. The population density was 2.3 inhabitants per square mile (0.89/km2). There were 2,002 housing units at an average density of 0.9 units per square mile (0.35 units/km2). The racial makeup of the county was 74.9% American Indian, 21.0% white, 0.2% Asian, 0.1% black or African American, 0.2% from other races, and 3.6% from two or more races. Those of Hispanic or Latino origin made up 1.8% of the population. In terms of ancestry, 13.2% were German, 6.5% were Irish, and 0.6% were American.
Of the 1,730 households, 45.9% had children under the age of 18 living with them, 37.6% were married couples living together, 24.2% had a female householder with no husband present, 28.4% were non-families, and 24.6% of all households were made up of individuals. The average household size was 3.05 and the average family size was 3.60. The median age was 30.0 years.
The median income for a household in the county was $33,255 and the median income for a family was $40,500. Males had a median income of $33,942 versus $28,594 for females. The per capita income for the county was $15,632. About 20.5% of families and 30.5% of the population were below the poverty line, including 39.5% of those under age 18 and 13.5% of those age 65 or over.
The county is divided into two areas of unorganized territory: North Dewey and South Dewey.
Politics
Dewey has since the 1990s been a strongly Democratic county in solidly Republican South Dakota. The last Republican to carry the county was Ronald Reagan in his 1984 landslide when he came within 3,761 votes of claiming all fifty states. Before this period, by contrast, Dewey was a Republican-leaning county even for South Dakota. Between its formation and 1984, Dewey had voted Democratic only in the three landslide Democratic wins of 1964, 1936 and 1932, plus for Woodrow Wilson in 1916 when his anti-war policies had strong appeal in the West.
United States presidential election results for Dewey County, South Dakota[17]
