The headwaters of the Big Thompson River begin in Forest Canyon within Rocky Mountain National Park in Larimer County, Colorado. The river flows east through Moraine Park to the town of Estes Park. There it is held in Lake Estes by Olympus Dam before being released into the Big Thompson Canyon. The North Fork Big Thompson River also begins in Rocky Mountain National Park, on the northern slopes of the Mummy Range. This tributary flows east, through the town of Glen Haven, where it merges with the Big Thompson River in the town of Drake, in the Big Thompson Canyon.