 The county area was effectively reduced on February 28, 1861 by the US Congress, when it created Colorado Territory, taking all of the Beaver County area east of 109 degrees longitude.The county's boundary was enlarged on January 17, 1861 by addition of parcels from Millard, Sanpete, and St. Mary's counties.However, its west area was reduced by creation of the Nevada Territory on July 14, 1862 (this adjustment was altered on May 5, 1866 by action of the US Congress, effectively ceding all Beaver County area west of 114 degrees longitude to Nevada counties).