Transmountain diversions may single-handedly be the most controversial and divisive water issue in Colorado.

A transmountain diversion (TMD) removes water from one river basin in the state and transports it across a mountain barrier into another river basin. This removal of water is not only legal, but the entitlement to do so is written into the state’s constitution.

Why is this done?

Colorado can be divided into two distinct halves by the Continental Divide. The divide spans the length of Colorado, from its northern border with Wyoming south to New Mexico. Rain and snow that falls on the west side of this dividing line will run to the Pacific Ocean, and precipitation that falls on the east side of the divide heads in the direction of Atlantic Ocean.

The Western Slope, the lands west of the Continental Divide, receives the vast majority of Colorado’s moisture, approximately 80 percent, while the East Slope on the other side receives only 20 percent.

The problem is over four out of every five Coloradoans live on the much drier, eastern side of the Divide.

In order to sustain these large and growing populations in major East Slope cities such as Denver, Boulder, Colorado Springs, Pueblo and Aurora, these municipalities have come to rely on diverting water from the wetter Western Slope to the eastern half of the state through transmountain diversions projects.

One of the first transmountain diversion occurred around 1900 when enterprising and scrappy farmers from northeastern Colorado dug the Grand River Ditch to siphon off water from the Colorado River’s headwaters in what is now Rocky Mountain National Park by “just going over there and taking it.”

Today, 12 major transmountain water diversions are removing water from the Colorado River Basin.

These 12 diversions combine to remove between 450,000 to 600,000 acre-feet of water from the Colorado River basin per year, which is enough water to cover the Denver metro area from five feet to over six feet deep in water.

The loss of water out of West Slope counties affected by transmountain diversions can be as high as 65 percent of the streamflow being diverted away to the other side of the divide.

Since water is used multiple times as it flows downstream, removing water from a basin through transmountain diversions means the loss of water can be felt in multiple ways. When water is diverted out of a basin, the ability to use and reuse that water is lost completely to the basin of origin.

Transmountain diversions can lead to a loss of economic, recreational and aesthetic benefits and have negative impacts to water quality and animal habitat.

A number of West Slope agencies keep a vigilant eye on existing and proposed future transmountain water diversions. Among them are the Colorado River Water Conservation District, Northwest Colorado Council of Governments, as well as county governments and local West Slope water conservancy districts and agencies.