The drought of the 1930s was the impetus for the Colorado-Big Thompson Project.
Work started in 1938 and would span nearly two decades to complete.
The first project was the Green Mountain Reservoir on the Blue River. The water stored ran north into the Colorado River and is used to compensate for water that would be diverted to the Eastern Slope.
A significant year for the project was 1944 when work ended on the Alva B. Adams Tunnel, just over 13 miles long. It carried water under the Continental Divide.
Lake Granby, the largest reservoir in the system, stores Colorado River water during the spring runoff. A second project was the nearby Shadow Mountain Reservoir connected to Grand Lake by a short canal. The two bodies of water are nearly 90 feet higher than Lake Granby.
The Alva B. Adams Tunnel’s west portal is on the east side of Grand Lake which, incidentally, is the largest natural water body in Colorado.
After the spring runoff and to keep Shadow Mountain Reservoir and Grand Lake filled, a pumping station brings Lake Granby water up to their level.
Added in 1951-52 and on the west side of the Continental Divide is the Willow Creek Reservoir. A pumping station elevates the water 175 feet to a canal flowing into Lake Granby.
The 9 ½ -foot in diameter Alva B. Adams Tunnel drops 109 feet in its 13 miles, ending at the East Portal.
From a small lake at the East Portal, the water is carried via a siphon under Aspen Brook to the Rams Horn Tunnel and via a penstock, down to the Marys Lake power plant. This is a drop of 205 feet.
Running directly under the summit of Prospect Mountain, yet another tunnel and penstock delivers water to the Lake Estes power plant, a drop of 482 feet.
From Lake Estes, water flows east first through the Olympus Tunnel to the 5 ½ -mile long Pole Hill Tunnel.
Water is delivered to the top of a canal then to a penstock. It drops 815 feet to the Pole Hill power plant. From there, the water enters the 1 ¾ -mile-long Rattlesnake Tunnel, ending on the west side of Pinewood Lake. An intake on the east end of Pinewood Reservoir takes water through the Bald Mountain Tunnel to the penstock visible from Loveland.
Water is delivered to the Flatiron power plant at Flatiron Reservoir over 1,000 feet below.
This is where things get complicated.
During times of excess water, it is pumped up to Carter Lake, 277 feet higher.
Water also flows through a short tunnel north to the Hansen Feeder Canal to Horsetooth Reservoir.
From the south end of Carter Lake, water is delivered into the South St. Vrain Supply Canal. This long canal takes water under part of Rabbit Mountain all the way the Boulder Reservoir.
In all, West Slope water drops nearly 3,000 feet during its journey to the East Slope.
The Colorado-Big Thompson Project has created a dozen reservoirs, uses 35 miles of tunnels and also generates a substantial amount of electric power. These are the power plants: