Shadow Mountain Lake is a reservoir

Reservoir

A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

 in Grand County, Colorado

Grand County, Colorado

Grand County is the 21st largest of the 64 counties of the State of Colorado of the United States. The county population was 12,442 at U.S. Census 2000...

, near the headwaters of the Colorado River

Colorado River

The Colorado River , is a river in the Southwestern United States and northwestern Mexico, approximately long, draining a part of the arid regions on the western slope of the Rocky Mountains. The watershed of the Colorado River covers in parts of seven U.S. states and two Mexican states...

. Shadow Mountain Lake is created by Shadow Mountain Dam

Shadow Mountain Dam

Shadow Mountain Dam is a zoned earthfill dam on the Colorado River in Grand County, Colorado. Constructed between 1944 and 1946, the Shadow Mountain Dam creates the Shadow Mountain Lake, with a structural height of and a drainage area of . Shadow Mountain Lake is a holding reservoir for water...

. The lake forms a continuous body of water with Grand Lake

Grand Lake (Colorado)

Grand Lake is Colorado's largest and deepest natural lake, one of the headwaters of the Colorado River in Grand County, Colorado. On its shores is located the eponymous town of Grand Lake. The elevation of the lake surface is 8,367 feet . It is formed by the damming of several streams by a...

, the largest natural lake in Colorado; they are separated by gates.

Shadow Mountain Lake is part of Arapaho National Recreation Area

Arapaho National Recreation Area

The Arapaho National Recreation Area is a United States National Recreation Area that is located near the headwaters of the Colorado River in north central Colorado adjacent to Rocky Mountain National Park. ANRA is under the jurisdiction of the Arapaho-Roosevelt National Forest...

.

The creation of Shadow Mountain Lake was authorized by the Colorado-Big Thompson bill, passed in 1937, which also created the Colorado-Big Thompson Project

Colorado-Big Thompson project

The Colorado-Big Thompson Project is a water diversion project in Colorado designed to collect West Slope mountain water from the headwaters of the Colorado River and divert it to Colorado's Front Range and plains...

. Water from the Grand Lake-Shadow Mountain Lake body is diverted through the Alva B. Adams tunnel

Alva B. Adams Tunnel

The Alva B. Adams Tunnel is the principal component of the largest transmountain water project in Colorado--the Colorado-Big Thompson Project . The tunnel transfers water from the western slope of the Colorado River drainage to users on the eastern Front Range of Colorado. The tunnel is long and...

, at the eastern edge of Grand Lake, into the Big Thompson River

Big Thompson River

The Big Thompson River is a tributary of the South Platte River, approximately 78 miles long, in the U.S. state of Colorado.- Course of the river :...

 on the east side of the Continental Divide

Continental Divide

The Continental Divide of the Americas, or merely the Continental Gulf of Division or Great Divide, is the name given to the principal, and largely mountainous, hydrological divide of the Americas that separates the watersheds that drain into the Pacific Ocean from those river systems that drain...

. Storage began in 1947.