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Arapahoe County was named for the Arapaho Indians who had inhabited eastern Colorado. Originally, the county extended all the way to the Kansas/Colorado border. November, 1858 Denver citizens formed the independent government of Arapahoe County within Kansas Territory and elected a delegate.

Colorado Territory was established by Congress on February 28, 1861. The first Legislative Assembly of the Territory of Colorado convened on September 9, 1861. They quickly enacted laws establishing the seventeen original counties.  They were Arapahoe, Boulder, Clear Creek, Costilla, Douglas, El Paso, Fremont, Gilpin, Guadalupe (later named Conejos), Huerfano, Jefferson, Lake, Larimer, Park, Pueblo, Summit and Weld.

Denver was Arapahoe’s County Seat in 1861

In 1902, Arapahoe county split and created Denver and Adams Counties, and sent parts of Arapahoe County to Washington and Yuma Counties.

1904 Littleton defeats Englewood, 1,310 votes to 829, in an election to determine the permanent Arapahoe County seat.

1913 - Denver - Colorado swore in its first woman elected to serve in the State Senate. She was Helen R. Robinson, R - Arapahoe County.

1910 Census: Arapahoe County Population = 10,263
1920 Census: Arapahoe County Population = 13,766
1930 Census: Arapahoe County Population = 22,647
1940 Census: Arapahoe County Population = 32,150
1950 Census: Arapahoe County Population = 52,125
1960 Census: Arapahoe County Population = 113,426
1970 Census: Arapahoe County Population = 162,142
1980 Census: Arapahoe County Population = 293,621
1990 Census: Arapahoe County Population = 391,511
2000 Census:  Arapahoe County Population = 487,967
2001 Estimated Arapahoe County Population = 500,785

Land Area = 803 square miles (year 2000)

Population Density = 607.6 people per square mile (year 2000)

Census information source: http://www.census.gov/population/cencounts/co190090.txt and http://quickfacts.census.gov/qfd/states/08/08005.html
Land area and population density source: http://quickfacts.census.gov/qfd/states/08/08005.html