Neither Spain nor Mexico established permanent settlements in the San Juan River country, due to the harsh winter weather and hostile native population. In 1848, after the Mexican–American War the area became part of the United States; prospectors arrived in the late 1850s and upon their discovery of placer gold in tributaries of the upper San Juan River a gold rush began to the San Juan Mountains. The gold rush reached its peak after the creation of Colorado Territory in 1861, bringing thousands of fortune seekers to this remote region. Silver was discovered along the Animas River in 1871 and led to the growth of present day Silverton below the area's labor intensive hard rock mines. The narrow-gauge San Juan Extension of the Denver and Rio Grande Western Railroad (D&RGW) was completed in 1881, linking Durango with the rest of the D&RGW system at Alamosa, Colorado via tracks through the upper San Juan River canyon.