The west boundary line of Sully County is defined by the meanderings of the Missouri River, which flows southward along its edge. The county's terrain is composed of semi-arid rolling hills, partially devoted to agriculture.[5] The terrain slopes to the south and east, but the west portion of the county slopes westward into the river valley. The county's highest point is along the midpoint of its north boundary line, at 1,949 ft (594 m) ASL.[6] The county has a total area of 1,070 square miles (2,800 km2), of which 1,007 square miles (2,610 km2) is land and 63 square miles (160 km2) (5.9%) is water.[7]
The eastern portion of South Dakota's counties (48 of 66) observe Central Time; the western counties (18 of 66) observe Mountain Time. Sully County is at the western edge of those counties that observe Central Time.[8]
U.S. Decennial Census[10] 1790–1960[11] 1900–1990[12] 1990–2000[13] 2010–2020[1]
2020 census
As of the 2020 census, there were 1,446 people, 635 households, and 428 families residing in the county.[14] The population density was 1.4 inhabitants per square mile (0.54/km2). There were 892 housing units.
2010 census
As of the 2010 census, there were 1,373 people, 610 households, and 397 families in the county. The population density was 1.4 inhabitants per square mile (0.54/km2). There were 845 housing units at an average density of 0.84 units per square mile (0.32 units/km2). The racial makeup of the county was 96.6% white, 1.2% American Indian, 0.1% black or African American, 0.0% from other races, and 2.2% from two or more races. Those of Hispanic or Latino origin made up 0.9% of the population. In terms of ancestry,
Of the 610 households, 27.0% had children under the age of 18 living with them, 57.5% were married couples living together, 4.6% had a female householder with no husband present, 34.9% were non-families, and 31.6% of all households were made up of individuals. The average household size was 2.25 and the average family size was 2.82. The median age was 46.6 years.
The median income for a household in the county was $48,958 and the median income for a family was $58,875. Males had a median income of $34,375 versus $29,087 for females. The per capita income for the county was $26,596. About 4.4% of families and 8.0% of the population were below the poverty line, including 12.8% of those under age 18 and 8.4% of those age 65 or over.
The county organization does not include division into townships. Its area is divided into two areas of unorganized territory: West Sully and East Sully.
Politics
Throughout its history, Sully County has been powerfully Republican. The solitary Democrat to carry Sully County at a Presidential level has been Franklin D. Roosevelt in 1932 during an election heavily influenced by the "Dust Bowl" and Great Depression. Nonetheless, in the following 1936 election, Alf Landon won the county by over twenty percent. Since that time, the Democratic Party has bettered FDR's 1936 effort five times, but only Lyndon Johnson in 1964 has held the GOP to a single-digit margin. In modern times, like almost all of rural America, Sully County has become more and more Republican. The last Democrat to carry one-third of the county's vote was Michael Dukakis in 1988 during an election severely affected by a major drought.
United States presidential election results for Sully County, South Dakota[15]
