Big Thompson River, CO

The Big Thompson River is a tributary of the South Platte River, approximately 78 miles (123 km) long, in the U.S. state of Colorado.

The headwaters of the 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 the Olympus Dam before being released into the Big Thompson Canyon. The North Fork of the 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.

From Lake Estes, the River descends 1/2 mile (800 m) in elevation through the mountains in the spectacular 25 mi (40 km) Big Thompson Canyon, emerging from the foothills west of Loveland. It flows eastward, south of Loveland across the plains into Weld County and joins the South Platte approximately 5 mi (8 km) south of Greeley. It receives the Little Thompson River approximately 4 mi (6 km) upstream from its mouth.