Shadow Mountain Lake near Grand Lake, Colorado was impounded in the 1940s as a component of the mammoth Colorado-Big Thompson project. The National Park Service managed a recreation area focused on the reservoir for nearly three decades before the task was transferred to the Forest Service in 1979. It’s now managed as part of the Arapaho National Recreation Area.

Curiously, the man-made water body called Shadow Mountain Lake wouldn’t exist at all if surface water weren’t so scarce on the eastern side of the Rocky Mountains. Because this mountain range is high, north-south oriented, and situated in a zone of prevailing westerly winds, it serves as a formidable obstacle to the eastward movement of moisture-bearing winds from the Pacific Ocean. The high western slopes of the mountains act like a ramp, forcing eastward-bound maritime air currents to rise, cool, and form thick cloud masses. The result is lots of rain and snow, lush forests, and most importantly for the West Slope economy, plenty of surface water. The Colorado River, the only major source of surface water for much of the arid Southwest, is born here.

A winter with heavy snowfall is a special blessing. The snowpack that serves as a playground for skiers and snowmobilers melts in the summer, steadily feeding streams that supply water to cities and towns, ranches, farms, mines, and resorts.

Water-needy enterprises situated east of the Rockies are not so lucky. By the time those eastward-moving air masses overtop the Front Range and descend the East Slope to the piedmont and plains, they have already been stripped of most of their moisture and are warming due to compression. Since this is the opposite of what you need for cloud formation, the result is a huge “rain shadow” cast over the lee side of the mountains. If it weren’t for the passing low pressure systems drawing moist Gulf of Mexico air into the region, true desert conditions would prevail. And if it weren’t for a gigantic diversion project that brings water from the other side of the Continental Divide, hundreds of square miles of productive land on the eastern side of the Rockies would be water-starved and bereft.

More than a century ago, big thinkers in rain shadow communities had already begun to dream of tapping into the surplus water on the western side of the Continental Divide. Why should that precious water on the West Slope just flow off through wilderness, desert, and the Grand Canyon when it could be used to grow crops, produce livestock, generate hydroelectricity, and support cities and industries on the dry East Slope?

Conceptually speaking, you could cure this problem by collecting water on the West Slope, transporting it under the Continental Divide via a tunnel, and storing it in reservoirs on the East Slope for use as needed. You would be able to generate copious amounts of hydroelectric power in the bargain. Of course, you would have to talk the West Slope folks into giving up some of their water, but you could do that by agreeing to construct reservoirs and water-related facilities supporting tourism, irrigated agriculture, ranching, mining, logging, and other industries important to the regional economy. You could, in other words, make it a win-win game.

The dream finally became reality with the completion of the Colorado-Big Thompson Project (C-BT). Constructed between 1938 and 1957, the C-BT now annually transports about 213,000 acre-feet of water from the headwaters of the Colorado River eastward through the Front Range and dumps it into the Big Thompson River, a major tributary of the South Platte. Stored on the East Slope in the Carter Lake, Horsetooth, and Boulder Reservoirs, this water is then used to supplement the meager natural surface water resources of the South Platte River basin. The supplemental water and related hydroelectric power serve municipal, agricultural, and industrial needs over a huge area of northeastern Colorado that encompasses 693,000 acres of irrigated farmland as well as over two dozen cities and towns.

The Colorado-Big Thompson Project was built to mammoth scale, eventually spanning 150 miles east to west and 65 miles from north to south. Building the C-BT required more than 100 structures, some of them quite large and expensive. A dozen reservoirs collect water that is distributed with the help of 35 miles of tunnels and 95 miles of canals. Five hydroelectric power plants harness the huge head of water represented by the half-mile drop in elevation, feeding electricity into 700 miles of transmission lines. Even people who criticize the project’s adverse environmental impacts concede that it is an engineering masterpiece.

The 13.1 mile-long Alva B. Adams Tunnel transports West Slope water under the Continental Divide and delivers it to the Big Thompson River and its reservoirs on the East Slope. The source of the water is the West Slope Collection System, three “sister lakes” situated in the headwaters of the Colorado River.

As the accompanying map shows, the water that enter the western portal of the Adams Tunnel comes directly from Grand Lake, the largest and deepest natural water body in the state of Colorado. Contiguous with Grand Lake is a reservoir called Shadow Mountain Lake, below which there is an even larger man-made body of water called Granby Lake. Critical to the functioning of the West Slope Collection System (better known as the Three Lakes locale) is the fact that Granby Lake water is pumped into Shadow Mountain Lake where it can be used to help maintain Grand Lake at the nearly constant level required by law. Grand Lake’s water level (normal elevation 8,367 feet) is not supposed to drop by more than one vertical foot while water is being diverted into the Alva B. Adams Tunnel.

The Shadow Mountain Dam was completed by the Bureau of Reclamation in 1946 at a site about 12 miles north of Granby. The 63-foot high earthen dam impounds an elongated reservoir with just eight miles of shoreline and 1,346 surface acres. Having a maximum depth of only 24 feet, Shadow Mountain Lake is shallower than its two sister lakes and holds less water. In fact, the lake’s active capacity of 16,848 acre-feet represents considerably less than a two day supply for the Adams Tunnel. Shadow Mountain Lake amply illustrates that a reservoir need not be huge to be hugely important.

Even before the Shadow Mountain Dam was constructed, it was abundantly clear that Shadow Mountain Lake and its shoreline would be a resource of great scenic, recreational, and ecological value. Because the Bureau of Reclamation was in the business of building dams, not managing scenery, recreational facilities, and wildlife habitat, the agency was quite happy to turn the latter functions over to the National Park Service on June 27, 1952.

Shadow Mountain Recreation Area remained a very lightly developed National Park System component for the better part of three decades before finally being transferred to the U.S. Forest Service by an act of Congress that took effect on March 1, 1979. The previous year, Congress had created a 36,000-acre water and forest wonderland called the Arapaho National Recreation Area. The ANRA featured five major lakes in the upper Colorado River Valley -- Lake Granby, Shadow Mountain Lake, Monarch Lake, Willow Creek Reservoir, and Meadow Creek Reservoir. (Area tourism promoters like to call these five ANRA lakes, plus adjacent Grand Lake, “the Great Lakes of Colorado.") Since Shadow Mountain Recreation Area was focused on a man-made water body, lacked nationally significant resource values, and could not be simply folded into Rocky Mountain National Park, it made a lot of sense for Congress to turn the management of this property over to the agency administering the newly established Arapaho National Recreation Area.

Postscript: Today the principal recreational activities on and near Shadow Mountain Lake are power boating and jet skiing, water skiing, sailing, fishing (brown trout, rainbow trout, and kokanee salmon), camping, hiking, wildlife watching, and mountain biking. Winter offers ice fishing, snowmobiling, cross country skiing, snowshoeing, and related winter sports. Although the nearby town of Grand Lake offers urban amenities, the Shadow Mountain Lake shoreline offers little in the way of recreation facilities. There are two boat launching ramps, an 80-site campground, and some hiking trails. On the plus side, the backcountry and wilderness character of the area has remained largely intact. Hikers and backpackers especially appreciate the links to the national forest’s trail system, nearby Rocky Mountain National Park, the Continental Divide National Scenic Trail, and higher-elevation wilderness areas extending from central Colorado all the way to the Wyoming border and beyond.