Continental Divide

The Continental Divide — also known as the Great Divide — is one of the defining geographic features of Colorado, running in a serpentine course along the crest of the Rocky Mountains from the Wyoming border in the north to the New Mexico border in the south. It is a hydrological boundary separating rivers that drain westward into the Pacific Ocean from those that drain eastward into the Atlantic Ocean and Gulf of Mexico. More than merely a topographic line on a map, the Divide shapes the state's climate, hydrology, settlement patterns, and economy — and has drawn explorers, miners, engineers, and outdoor enthusiasts for centuries.

Geography and Physical Character

The Continental Divide of the Americas is the principal, and largely mountainous, hydrological divide of the Americas, extending from the Bering Strait to the Strait of Magellan and separating the watersheds that drain into the Pacific Ocean from those that drain into the Atlantic and Arctic Oceans, including the Gulf of Mexico, the Caribbean Sea, and Hudson Bay. Within the United States, the Divide crosses into Wyoming within Yellowstone National Park and continues southeast around the Great Divide Basin, through the Sierra Madre Range into Colorado, where it reaches its highest point in the contiguous United States at the summit of Grays Peak at 4,352 metres (14,278 ft).

From north to south, the Great Divide in Colorado follows a serpentine path through Routt, Jackson, Larimer, Grand, Boulder, Gilpin, Clear Creek, Summit, Park, Lake, Eagle, Pitkin, Gunnison, Chaffee, Saguache, Hinsdale, San Juan, Mineral, Rio Grande, Conejos, and Archuleta Counties. The Divide forms part of the boundary of each county it traverses.

Entering Colorado from Wyoming in the north, the Continental Divide stays above 10,000 feet and passes through mountaintops exceeding 12,000 feet before dropping to 9,426 feet at Rabbit Ears Pass. The altitude and environment of the Divide range from high alpine tundra at places like Grays Peak and the flanks of San Luis Peak (14,022 ft) to lower-elevation basins, such as the area just east of Rabbit Ears Pass at the southern end of North Park. In most parts of the state, the route of the Divide is flanked by coniferous forests and inhabited by a variety of flora and fauna, including marmots, bighorn sheep, and elk.

Within the state, the area east of the Great Divide is generally referred to as the Eastern Slope — or the Front Range — while the area west of the divide is known as the Western Slope. The Rocky Mountains, which form much of the Great Divide, began rising over 70 million years ago due to tectonic activity, directing rivers toward either the Pacific or Atlantic Oceans.

Climate and Ecology

The Continental Divide functions as a major orographic barrier, forcing moisture-laden Pacific air masses upward as they move eastward, causing heavy precipitation on the Western Slope and significantly drier conditions on the Eastern Slope. This pronounced rain shadow effect means that communities on Colorado's Western Slope receive considerably more annual snowfall than comparable elevations on the east side of the Divide. At the highest elevations along the Divide, persistent snowfields and extensive alpine tundra dominate the landscape, giving way at lower elevations to subalpine forests of Engelmann spruce and subalpine fir, and then to montane zones characterized by lodgepole pine, aspen, and ponderosa pine.

The alpine tundra communities found along the Divide's high ridgelines are among the most ecologically fragile environments in Colorado. Short growing seasons, intense ultraviolet radiation, and fierce winds restrict plant life to low-growing cushion plants, sedges, and grasses. Wildlife along the Divide reflects this full range of elevational zones. Pika and yellow-bellied marmots are common residents of talus fields near the summits, while bighorn sheep, mountain goats, elk, mule deer, and black bears range across the subalpine and montane slopes below. Golden eagles and white-tailed ptarmigan are among the more distinctive bird species associated with the Divide's high terrain.

Human History

Humans have relied on the high-altitude resources of Colorado's Great Divide for millennia. Beginning around 1400, Ute people lived and hunted along the Divide, making seasonal treks across its many passes and forging trails that later became the routes of railroads and highways. Before modern settlers, tribes including the Utes undoubtedly crossed the Divide in Colorado during the summer months, and many of their pathways through the Rocky Mountains pre-date the modern routes over the same mountain passes.

The first modern settlers and explorers to cross the Continental Divide did not come through Colorado. Lewis and Clark crossed the Continental Divide at Lemhi Pass in Montana in 1805, and the Oregon Trail crossed the Divide over South Pass in Wyoming. The terrain over South Pass would be considered mild compared to any path over the Divide in Colorado. Zebulon Pike's 1806 expedition into the southern Rockies, while failing to summit the peak that bears his name, brought early American attention to the mountains that define Colorado's spine. John C. Frémont's subsequent surveys in the 1840s helped map the passes and river drainages that would guide later settlement and commerce.

Tennessee Pass is an important pass historically. The area was explored by John C. Frémont and Kit Carson in 1845, as the nearby state historic plaque attests. Also at the pass is the Ewing Ditch, which was among the first trans-continental water diversions on record, and may be the oldest one still in use. Constructed in 1880, it extends for a mile up the side of Piney Gulch, capturing drainage that would otherwise go into the Colorado River, and moves it over the Divide where it drains into the Arkansas River watershed.

The discovery of gold near Denver in 1858 and subsequent silver strikes throughout the mountains transformed the Divide from a geographic curiosity into an economic frontier. Mining camps sprang up on both slopes, and the need to move ore, supplies, and people across the mountains efficiently drove an era of ambitious road and railroad construction. Many wagon roads were built over the Divide during the late 1800s. Some of these were eventually turned into modern auto roads, including Highway 6, the predecessor to Interstate 70 and the Eisenhower Tunnel. Toll roads played an important role in Colorado's development, and many of today's mountain passes began as toll roads that were used by wagons to haul mining ore out of the Rockies.

In 1882, a toll road was built over Independence Pass, though traffic dropped off when the railroads reached Aspen a few years later. In the 1920s, the State of Colorado started maintaining the road as State Highway 82. The twentieth century saw the Great Divide's status as a formidable barrier progressively reduced, as Coloradans and the federal government used new technology and advanced engineering to bore through its granite flanks and create tunnels for rails, roads, and water.

Hydrology and Water Diversion

Most of Colorado's water, provided by snowmelt, spills into the western side of the Divide, while most of the population lives on the eastern side. This imbalance has led to water diversion projects that move water from one side of the Divide to the other, dating back more than a century, and include some of the largest tunneling projects in the country.

One historic dilemma the state faced was the fact that roughly 80% of the state's water originates on the Western Slope, while approximately 80% of the population lives on the Eastern Slope, where cities and farms require reliable water supplies. By building a series of reservoirs, canals, and tunnels that pass under the Continental Divide, trans-mountain diversions were created to send water east. Sixty to seventy percent of Colorado Springs' water arrives that way, and Denver and many other cities and towns along the Front Range are also heavily reliant on Western Slope water.

The Great Divide in Colorado is the source of the Colorado, Rio Grande, and Arkansas Rivers, three of the continent's most important waterways. It also contains the headwaters of the Colorado River's major tributaries, including the Gunnison and San Juan Rivers. On the eastern side of the Divide, the South Platte River gathers its headwaters from the high country around South Park before flowing northeast toward the Colorado plains, while the Arkansas River begins near Leadville before carving its dramatic course through Royal Gorge.

In 1922 the state legislature passed a bill to create a 6.2-mile tunnel through the Great Divide for the Denver, Northwestern & Pacific Railway — a project long planned as part of businessman David H. Moffat's rail line, nicknamed the Moffat Road. The Moffat Tunnel, as it came to be known, was completed in 1928, with its east portal located along South Boulder Creek west of Rollinsville and its west portal located in Winter Park. The tunnel also carries a water bore that diverts Western Slope water into the South Boulder Creek drainage on the eastern side.

Between the 1930s and 1950s, the Colorado–Big Thompson Project was completed, bringing water from the Colorado River on the Western Slope to farms and cities along the Front Range. The lynchpin of the project is the Alva B. Adams Tunnel, bored through the Continental Divide under Rocky Mountain National Park. Water passes through the tunnel on its way to a labyrinth of reservoirs, canals, and ditches along the Front Range. The Fryingpan-Arkansas Project, completed in phases during the 1960s and 1970s, similarly carries water from the Fryingpan River drainage on the Western Slope through the Divide to augment the Arkansas River basin.

The rivers may flow to the east and west on their respective sides of the Continental Divide, but thanks to human engineering, some of the precipitation that falls on the western side of the Divide in Colorado crosses the Divide at least once before reaching its destination on the eastern side. In the section of the Divide near Jones Pass and Berthoud Pass, water crosses the Divide three times before finally heading out of the mountains, passing through the Gumlick Tunnel, then the Vasquez Tunnel, then the Moffat Tunnel.

Mountain Passes

There are a total of 15 paved mountain passes that cross the Divide in Colorado. These passes have served as critical corridors for Native American travel, mining operations, rail lines, and modern highway traffic alike, and each carries its own distinct history shaped by the demands of the era in which it rose to prominence.

Loveland Pass crests at 11,990 feet on U.S. Highway 6 and remains one of the most heavily used year-round crossings in the state, offering a direct route over the Divide for commercial vehicles that cannot use the Eisenhower Tunnel below. The Eisenhower Tunnel — formally the Eisenhower-Johnson Memorial Tunnels — passes through the Divide at an elevation of approximately 11,000 feet on Interstate 70, providing a year-round passage that stands as a major engineering achievement of the 20th century. At the time of its completion in 1973, the Eisenhower Tunnel was the highest vehicular tunnel in the world. Berthoud Pass, carrying U.S. Highway 40 at 11,307 feet, was one of the primary routes connecting Denver to the Western Slope before the Eisenhower Tunnel opened and remains an important crossing for recreational travelers.

Independence Pass, the second highest paved crossing of the Continental Divide at 12,095 feet, is located ten miles south of the mostly disused tunnels of Hagerman Pass. The highway continues west to Aspen and is active with cyclists and tourists during the summer months. The modern highway is closed for half the year because of snow, though the old toll road was historically kept open year-round.

Just south of Breckenridge, Hoosier Pass sits at 11,542 feet and offers views across the Mosquito Range. A parking area at the summit makes it a convenient stop, and visitors can walk a short distance to stand directly on the Divide. Monarch Pass, at 11,312 feet on U.S. Highway 50, serves as the principal crossing for travelers moving between the San Luis Valley and the Gunnison country, and Wolf Creek Pass at 10,857 feet on U.S. Highway 160 is notable for receiving some of the heaviest snowfall of any paved pass in the state, routinely exceeding 400 inches annually.

The Continental Divide National Scenic Trail

Established by Congress in 1978, the Continental Divide National Scenic Trail (CDT) spans approximately 3,100 miles between Mexico and Canada, traversing five states and connecting countless communities along the spine of the Rocky Mountains. Colorado holds a substantial share of this iconic route, with 740 miles of the trail passing through the state.

The establishment of the Appalachian Trail and the Pacific Crest Trail inspired proposals to create a Continental Divide Trail. The first section of the proposed trail was laid out in Colorado in 1962 by the Rocky Mountain Trails Association. In 1965, President Lyndon Johnson proposed a national system of trails, and in 1968 the U.S. Congress adopted the National Trails System Act. In 1978, the Continental Divide Trail was formally established, with management responsibility assigned to the U.S. Forest Service.

The CDT in Colorado is dominated by the sharp, tall spine of the Rockies. The average elevation of the trail in Colorado is 10,000 feet above sea level, and along the way it summits Grays Peak — the highest peak on the Continental Divide in the contiguous United States and the highest elevation of any National Scenic Trail — at 14,270 feet above sea level. The alpine tundra and high-elevation forests of Colorado along the CDT are home to mountain goats, coyotes, pikas, black bears, and elk. The 740 miles of the CDT in Colorado are broken into nine segments, each offering its own distinct character and challenge.

Heading north from the border with New Mexico, the CDT soon enters the Weminuche Wilderness, Colorado's largest designated wilderness area, situated within the Rio Grande and San Juan National Forests. This roughly 100-mile section of the CDT is known for its ruggedness and isolation, traversing remote terrain that sees far fewer visitors than the trail's more accessible northern segments. Continuing northward, the route crosses the La Garita Wilderness, passes near the headwaters of the Rio Grande, and traverses the high country of the Cochetopa Hills before entering the dramatic San Isabel and Gunnison National Forests.

Ranging from 4,000 to 14,000 feet in elevation, the CDT provides a variety of recreational opportunities to hundreds of thousands of people each year, including hiking, horseback riding, cross-country skiing, snowshoeing, wildlife viewing, fishing, hunting, sightseeing, mountain bicycling, backpacking, and more. Thru-hikers who complete the Continental Divide Trail, the Appalachian Trail, and the Pacific Crest Trail achieve what is known as the Triple Crown of Hiking.<ref>{{cite web |title=Explore the Continental Divide Trail |url=https://cdtcoalition.org/explore-the-trail/ |work=Continental