This post is the first in a series fulfilling requirements for ENVS 5000: Policy, Science and the Environment, at the University of Colorado Boulder.
Perhaps the signature element of water resource management in Colorado is trans-basin diversions. As the name implies, these are artificial diversions of water from one drainage basin to another. In other words, they remove water from the river system it naturally flows through, sends it through a canal, tunnel, a pumping system (or some combination thereof), and deposits it on the other side of a mountain range or natural divide into a river system that it normally would never have entered. The implications of this strategy are both significant and wide-ranging to the hydrologic sciences, water policy, resource economics, and environmental ethics. In Colorado, the transfer of water from one basin to another is the source of much economic opportunity and much political strife.
This series of blog posts will explore transbasin diversions in Colorado, looking at their history, the science that underlies them, and their ethical, economic and political implications. We begin with how we got here: the history of transferring water across mountains in Colorado, and the reasons we feel the need to do so in the first place.
Why Move Water Under Mountains?
Colorado is a headwaters state: six major rivers (the North Platte, South Platte, Arkansas, Rio Grande, San Juan and the mighty Colorado herself) begin their lives within our borders. Almost no water flows into Colorado from surrounding states.
The above diagram, called the “snake diagram,” displays the average annual flow of rivers as they flow downhill from the Colorado Rockies to the borders of the state. Two things become readily apparent from this diagram: one, Colorado sends water to both the Pacific and Atlantic Oceans. The dividing point between the two watersheds is the Continental Divide; on the diagram, the areas marked 1, 2 and 3 (and part of 6) flow east into the Gulf of Mexico and the areas marked 4, 5, 7, and the remainder of 6 flow west into the Gulf of California.
The second thing that is apparent from the diagram is that by far the most water is located on the western side of that divide. Even after transbasin diversions, the amount of water flowing west is almost eight times the water flowing east. This is because by far more precipitation falls on the western half of the state, as shown by the following map.
Since most of Colorado’s precipitation falls as winter snow, and most of Colorado’s high mountains are west of the divide, most of the water will flow to the Colorado River and Gulf of California if we never interfered. This one of the two key facts that lead to transbasin diversions in the state: simply put, the water is on the west side.
The second fact has to do with water users. Below is the population of Colorado by county. Darker colors indicate more people.
This map makes the second fact relevant: most of Colorado’s population is located in the corridor from Larimer to Pueblo counties, what Coloradans call the “Front Range.” Put more simply, the people are on the east side. As of 2000, the eastern slope (the South Platte and Arkansas basins) had 80% of the state’s population and only 16% of its natural water. With the disparity of water supply and water users, there developed a long tradition of moving the water from west to east.
Population is not an exact proxy for water demand, and nothing like 80% of the state’s water gets transferred across the Continental Divide. In fact, less than 600,000 acre-feet moved underneath the Divide in 2000 (see page 3 of the previous link), out of nearly 10 million acre-feet leaving the state.
One Hundred Fifty Years of Parting the Waters
The oldest water right in Colorado (the first recorded beneficial use) is from 1852. The first transbasin diversion was constructed just eight years later. According to the State Engineer, there are thirty-two transbasin diversion in Colorado, including twenty-nine that cross the Continental Divide:
The oldest ditch on the surviving list, the second one constructed in Colorado, and one of the best-known, is the Grand Ditch, located within the confines of Rocky Mountain National Park. The Grand Ditch diverts water from the very headwaters of the mighty Colorado River and sends it eastward into the Cache la Poudre system. This drains as much as 40% of the uppermost waters of the Colorado, with attendant impacts on stream morphology and ecosystems on the west side of the park. Transbasin diversions are often much larger than this system, and the scale of potential impacts (and controversy) necessarily increases.
The largest users of transbasin water are the municipalities of the Front Range, especially the customers of Northern Water and Denver Water, and the farmers and ranchers along the lower South Platte River. Both entities believe that they will need to divert more water from the upper Colorado River system to the eastern slope to accommodate future water demands.
Since transbasin diversions are a “zero-return” system- by definition, none of the water they remove from the originating basin will return to it- their impacts to stream health, aquatic and riparian habitat, local agriculture, economies and society are far greater than an ordinary diversion within a basin. They serve as a point of conflict over the zero-sum game of water resource distribution. In Colorado, the players tend to be Front Range cities and eastern slope farmers against western slope farmers and environmental and recreation interests. Recently there has been a cautiously encouraging trend toward deliberation, cooperative planning, and the signing of agreements between interested parties, such as the Colorado River Cooperative Agreement between Denver Water, the state’s largest single user, and a coalition of western slope interests.
Whether that will be the norm or the exception remains to be seen. The implications of major transbasin diversions, as a zero-sum game concerning the most important resource in this arid state, on Colorado’s environment, politics, economy and society, are widespread. This series of blog posts will examine this phenomenon from a variety of perspectives.