Transbasin Diversions in Colorado: A History

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.

"Snake diagram" of stream  flows leaving Colorado

Average annual stream flows leaving Colorado as of 2011. Wider lines represent more flow (all values are in acre-feet). Image courtesy Colorado Division of Water Resources.

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.

Map of Colorado's average annual precipitation

Map of average annual precipitation. Warmer colors are drier (all units are inches per year). Image courtesy NOAA. Click the image for a full-size version.

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.

Map of Colorado's population by county

Colorado’s population by county as of 2010. Darker colors indicate more people. Image courtesy US Census Bureau.

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:

Map of Colorado's current transbasin diversions

Current transbasin diversions in Colorado grouped according to receiving basin. Image courtesy Colorado Division of Water Resources.

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.

Denver's skyline at night

The Denver Metropolitan Area and the other urban areas of the Front Range continue to outpace growth in the rest of Colorado. Photo: Larry Johnson

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 fromĀ  a variety of perspectives.


2 thoughts on “Transbasin Diversions in Colorado: A History

  1. Thank you for an interesting post on the history of trans-basin, surface-water diversions in Colorado. Your discussion and excellent map-displays caused me to consider a question which may appear, at first, off topic; but I promise to return to water diversions in the end.

    Doesn’t the “east-side” have a source of water which the “west-side” does not have: that being the subsurface aquifers of Tertiary formations like the Ogallala? And couldn’t it be economically, and possibly (intra-state) politically, more feasible to store additional volumes of surface flow from Colorado’s Atlantic watersheds for diversion (see how I got back on topic) to the subsurface reservoirs for future Front Range usage? Based on your “snake diagram” it appears that >500,000 AF of surface water is exiting the state through drainage basins 1 and 2. I note that the South Platte peaks at 818 thousand AF before declining to 384 AF on exiting the state. The Arkansas peaks at 666 thousand AF and exits at 155 AF. I hope that the approximately half-million AF difference from peak to exit in both cases is mostly from usage in plains farming – but some is probably just lost through evaporation.

    So, I’m wondering why the trans-basin solution which according to your numbers moves a similar water volume (you cite 600,000 AF) has become the preferred process? It is certainly not the easiest engineering solution nor the cheapest operation. One obvious answer is probably the difference in percentages lost from the natural systems. A half-million AF diverted from the western-slope flows is a small percentage of the total; whereas, that same volume diverted from the eastern rivers would essentially deplete all surface flows into Nebraska and Kansas – probably an ecological disaster and inter-state political nightmare. Still, if a large part of the difference from peak to exit is lost in evaporation (not impossible given your rainfall map…those are arid plains) then there could be compelling reasons to try and capture some of those waters into surface reservoirs with diversion into subsurface aquifers thereby limiting the evaporative losses and retaining these locally-derived waters for in-state usage along the populous Denver corridor without severe ecological or political impacts.

    If you can find time, I would like to hear what you think about this as an additional way to supply this valuable resource to the thirsty masses.


  2. Interesting questions PD. I think there are several reasons that we don’t see Colorado diverting something like 600 KAF for storage in subsurface aquifers on the east slope.

    The first is an engineering and economic reason (or set of reasons), which is that it probably actually IS easier to tunnel under (or over, in many cases) the Divide than to inject water into the formations under the Denver Basin, because the cost of building a transbasin diversion is largely a sunk cost rather than an ongoing marginal cost. Building the Adams or Roberts Tunnels were very expensive, sure, but water does not have to be pumped through them. I can’t seem to make a link in this comment box, but both of those tunnels (for Northern Water and Denver Water, respectively), operate on gravity alone. Reclamation even generates hydroelectric power on the East Slope side of the Adams Tunnel system as the water falls from the altitude of Estes Park to the altitude of Fort Collins. Trying to get 600 KAF into and out of the Denver Basin aquifers would require either EXTREMELY slow percolation through gravel beds (with attendant evaporative loss) or active injection, which carries an energy cost. I don’t know the exact situation of the various underlying formations under the Front Range, but I suspect all of them have an energy cost either in injection or extraction. Not every transbasin diverter does so without pumping (notably, Aurora and Colorado Springs), but Northern and Denver have been recovering the costs of their tunnels for several decades.

    The second reason (or set of reasons) is legal and political. As you noted, 600 KAF out of the South Platte and Arkansas Rivers would entail almost no flow going into Kansas and Nebraska, especially once you factor in the evaporative losses of gravel beds. Both of those rivers are governed by interstate compacts like the Colorado is. Again, I can’t make a link in comments but the Arkansas River Compact prohibits Colorado from “materially depleting” the river of flows that would be “generally available” to Kansas and the South Platte River Compact prohibits Colorado from depleting the river at the Nebraska line below a certain flow rate (which, presumably, would suffer greatly if 600 thousand additional AF were permanently removed from the channel).

    I hope that got at an answer to your questions, and if not, you’ll be pleased to hear that my next two posts in this series are about law and engineering.

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