Platte Canyon is the dramatic gateway where the South Platte River leaves Colorado’s Rocky Mountains, cutting through the Rampart Range and hogbacks before reaching the plains southwest of Denver. The name can refer either to a broader mountain stretch of the South Platte and North Fork South Platte or more narrowly to the lower gorge, often called Waterton Canyon. Between South Platte and Kassler, this steep, narrow passage drops about 600 feet in 8 miles. It became important in Colorado transportation when the Denver, South Park and Pacific Railroad built the first rail line through it in the 1870s. Later water projects reshaped part of the lower canyon, including Strontia Springs Reservoir.