The United States acquired the current area of Grand County via the Louisiana Purchase of 1803, but it was still controlled mostly by the Utes and Arapaho for several decades thereafter. Vast numbers of beaver and other fur-bearing animals brought fur trappers into Middle Park as early as the 1820s. Fur trappers had led hunting parties in the Grand Lake area since the 1820s, but by mid-century, they were building summer lodges by the lake.
The first permanent white residents, however, did not arrive until after the Colorado Gold Rush of 1858–59. Joseph L. Wescott became the first permanent resident of the area when he built his cabin on Grand Lake's west shore in 1867.
William N. Byers, the founder of Denver's Rocky Mountain News, came to Middle Park in 1860 and located the hot springs, pinched between two small mountains at the foot of the Rabbit Ears Range. He planned to turn the area into a resort community and founded the town of Saratoga West in 1860. Byers built a resort around the springs, and the town's name was changed to Hot Sulphur Springs in 1863.
In 1862, road builder Edward L. Berthoud made new inroads for white settlement in Middle Park when he surveyed a stagecoach route along a seldom-used Ute trail that is now known as Berthoud Pass.