The headwaters of the Big Thompson River begin in Forest Canyon within Rocky Mountain National Park in Larimer County, Colorado.

The North Fork Big Thompson River also begins in Rocky Mountain National Park, on the northern slopes of the Mummy Range.

From Lake Estes, the river descends 1⁄2 mile (800 m) in elevation through the mountains in the spectacular 25-mile (40 km) Big Thompson Canyon, emerging from the foothills west of Loveland.

[5] This flood was triggered by a nearly stationary thunderstorm near the upper section of the canyon that dumped 300 millimeters (12 inches) of rain in less than 4 hours (more than 3/4 of the average annual rainfall for the area).

The biggest infrastructure casualty, however, was the City of Loveland's hydroelectric plant (rebuilt after the 1976 flood); the Idylwilde Reservoir was completely filled with silt and rocks, the Idylwilde Dam broke free of the bedrock, and the hydroelectric plant in the Viestenz-Smith Mountain Park was filled with water and silt.