At a Glance
- Construction: 1938–1957
- Created: Shadow Mtn Lake, Lake Granby
- Water Transfer: CO River to East Slope
- Management: Bureau of Reclamation
Why the Project Was Built
The Colorado-Big Thompson Project was conceived to address a fundamental geographic challenge: the majority of Colorado's water flows west (toward the Colorado River) while the majority of Colorado's agricultural land is east (on the Front Range and plains). The project transfers water from the Colorado River watershed to the eastern slope through a system of tunnels, reservoirs, and canals.
Authorization came from Congress in 1937 and construction began in 1938. The project required excavating the 13.1-mile Alva B. Adams Tunnel through the Continental Divide — one of the longest water tunnels in the Western Hemisphere at the time of its completion — to carry Colorado River water under the mountain to the eastern slope.
Creating Shadow Mountain Lake and Lake Granby
The project created Shadow Mountain Lake by damming a section of the Colorado River south of Grand Lake. The Shadow Mountain Dam, completed in 1946, controls the lake level. Shadow Mountain Lake is connected to Grand Lake via a short diversion canal, allowing water movement between the two lakes.
Lake Granby was created by the Granby Dam on the upper Colorado River, completed in 1950. At over 7,000 acres, Lake Granby became Colorado's second-largest body of water and the primary storage reservoir for the C-BT project. Water is pumped from Lake Granby into Shadow Mountain Lake, then flows through Grand Lake and into the Adams Tunnel for transport to the eastern slope.
Recreation Legacy
The creation of Shadow Mountain Lake and Lake Granby transformed Grand Lake's recreation landscape. Where previously only the natural Grand Lake existed, the project added thousands of acres of water surface adjacent to the existing lake, creating the multi-lake recreation complex that defines the area today.
Two of Grand Lake Cabins' properties — Lakeside Cabin and Mid-Century Modern — overlook Shadow Mountain Lake. The reservoir's creation in the 1940s and 50s is directly responsible for the lake views that are central to the experience of staying in those properties.