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Transmountain diversions

Large-scale development of Colorado River water supplies started in the late 19th century, at the river's headwaters in La Poudre Pass. The Grand Ditch, directing runoff from the river's headwaters across the Continental Divide to arid eastern Colorado, was considered an engineering marvel when completed in 1890. This was the first of twenty-four "transmountain diversions" constructed to draw water across the Rocky Mountains as the Front Range corridor increased in population. These diversions draw water from the upper Colorado and its tributaries into the South Platte River, Arkansas River and Rio Grande basins. Today, about 80 percent of Colorado's population lives on the Eastern Slope of the Rockies, while 80 percent of precipitation falls on the Western Slope.

While first planned at the same time as the Grand Ditch, construction on the Colorado-Big Thompson Project (C-BT) did not begin until the 1930s. Today, the C-BT is the largest of the transmountain diversions, delivering 230,000 acre-feet (280,000,000 m) per year from the Colorado River to cities north of Denver. Numerous other projects followed, with the largest including the Roberts Tunnel, which delivers water from the Blue River to the city of Denver, and the Fryingpan–Arkansas Project, which diverts water from the Fryingpan River to the Arkansas River basin.

Combined, the transmountain diversions draw about 580,000 acre-feet (720,000,000 m) of water per year out of the Colorado River basin. Historically, most of the water has been used for irrigation, although water usage is increasing for urban water supply and for recreational purposes such as snowmaking and increasing Eastern Slope streamflows for boating and fishing. Denver Water receives about 50 percent of its supply from the Colorado River basin. However, diversions have caused environmental harm to the upper Colorado River system by reducing streamflows in many tributaries. A number of reservoirs have been built to offset the impact of transmountain diversions by storing water for dry season release on the Western Slope, including Williams Fork Reservoir in 1959 and Wolford Mountain Reservoir in 1996.