Dividing waters: How a compact call might unfold on Western Slope

Brent Gardner-Smith
Aspen Journalism

Coloradans use, or deplete, an annual average of 2.5 million acre-feet of water from the state’s various river basins that send water down the Colorado River system to Lake Powell, according to a new study on water use and water rights presented Thursday in Grand Junction by the Colorado River District to more than 130 water managers and users.

Of the 2.5 million of annual average consumptive use, the study found that 1.6 million is tied to pre-Colorado River Compact water rights that are not subject to curtailment under the terms of the 1922 compact, which requires that the upper-basin states of Colorado, Utah, Wyoming and New Mexico send a certain amount of water to the lower-basin states of California, Arizona and Nevada.

But that leaves 932,000 acre-feet of current consumptive water use that is subject to compact curtailment, because the use of the water is tied to post-compact water rights.

The study, which drills down into water use in the various river sub-basins on the Western Slope, found that of the 932,000 acre-feet of post-compact water use, 626,200 acre-feet is from the Colorado River Basin, which includes the main stem of the Colorado and the tributaries that drain the Roaring Fork, Eagle, Blue and Fraser river basins in Pitkin, Eagle, Summit and Grand counties.

The Colorado River Basin, for the purposes of water-rights administration, is defined by the state as Division 5. And it accounts for virtually all of the transmountain diversions from the Western Slope to the Front Range.

Of the 626,200 acre-feet of average annual post-compact use in the Colorado River basin, 532,000 acre-feet is attributed to transmountain diversions.

This isn’t breaking news for Front Range water managers, who have long known that their water from the Western Slope is based on post-compact water rights.

“I would be very surprised if any of this is a surprise to them,” said John Carron of Hydros Consulting Inc. in Boulder. He is the engineer managing the study and its complex hydraulic model.

But the numbers in the study showing the high reliance on post-compact water rights help illustrate how intertwined the Western Slope and the Front Range are in the face of a compact call.

“This really is a statewide issue, not just a West Slope issue, just by virtue of the fact that 60% of post-compact use in Colorado is by transmountain users,” John Currier, the river district’s chief engineer, said at Thursday’s meeting.

The study also sought to determine whether future depletions from the larger Colorado River upper-basin river system would increase the likelihood of a compact call, based on the hydrology recorded from 1988 to 2015.

The model used in the study found that if new depletions increased by 12%, it would increase the likelihood of compact call by 46% — if the goal was to deliver 82.5 million acre-feet a year to the lower basin on a 10-year running average. If the goal was to deliver 75.5 million acre-feet over 10 years, the model indicated a 0% increase in risk, primarily because of a new agreement to use reservoirs upstream of Lake Powell, mostly Flaming Gorge Reservoir, to send water downstream.

But, Carron noted, “all models are wrong, some are useful,” and said the risk of a compact call could be higher if there is less water in the future than the amount used in the model.

The study also asked if a compact call were to materialize, how far down a list of post-compact water rights, prioritized by dates, would the state have to curtail in order to send significant amounts of water downstream?

The study found that if the state needed to send 100,000 acre-feet downriver to the lower basin, it would have to curtail post-compact water rights dating to July 1957.

That’s a notable date, because the water rights for the Fryingpan-Arkansas Project — which diverts about 52,000 acre-feet a year from the headwaters of the Fryingpan and Roaring Fork rivers — have a 1957 priority date and could be curtailed early in the process.

If the state needed to send 300,000 acre-feet downriver, it would need to curtail rights dating to September 1940, and that could curtail Denver Water’s diversions from Dillon Reservoir through the Roberts Tunnel, which are made under water rights with a 1946 priority date.

If the state needed to send 600,000 acre-feet, it would require curtailing post-compact rights dating to August 1935, which could curtail the rights from 1935 that allow water to flow through the Adams Tunnel to northern Colorado.

The study also provides an overall portrait of how Coloradans are using water from the Western Slope basins.

Of the 2.5 million acre-feet of water that Coloradans are now using from the state’s rivers that send water down to Lake Powell, including use tied to both pre- and post-compact rights, 1.2 million acre-feet is being depleted from the Colorado River Basin, or Division 5.

Of that 1.2 million acre-feet, 669,000 acre-feet is going toward in-basin uses, such as irrigation in the Grand Valley, and 551,000 acre-feet is going toward transmountain diversions.

Depletions in the other Western Slope basins include 552,000 acre-feet from the Gunnison River basin; 501,000 acre-feet from the Southwest basin, which includes the Dolores and San Juan rivers; 197,000 acre-feet from Yampa River basin; and 62,000 acre-feet from the White River basin.

In each basin, most of the water being depleted from the rivers is going to grow hay, grass and alfalfa, with smaller portions being used to irrigate orchards and row crops, and still smaller portions being used to provide water to people and their lawns in cities and towns.

Most water users in Colorado are now considering ways to use less water, and most also are concerned about new uses in the system, which could hasten a compact call.

But a specific concern for the Western Slope arises from the Front Range’s heavy reliance on post-compact transmountain diversions, which account for about 60% of Front Range use.

Front Range water managers seem to have two options in the face of a compact call: Buy up land on the Western Slope with senior water rights, and then fallow the land and send the water downstream; or ask the state to not strictly apply the priority system, based on the dates of water rights, in order to avoid junior, post-compact, rights being curtailed.

Either way, the reliance by the Front Range on post-compact water rights is expected to ripple right back to the Western Slope.

“It’s time to start talking with our colleagues on the Front Range,” Jim Pokrandt, the river district’s director of community affairs and the chair of the Colorado Basin Roundtable, said at the conclusion of Thursday’s meeting.

Barbara Biggs, the chair of the Metro Basin Roundtable, which includes the Denver metro area, was at Thursday’s meeting, and she concurred with Pokrandt.

“Talking is always better than not talking,” Biggs said. “And planning is always better than not planning.”

Aspen Journalism covers rivers and water in collaboration with The Aspen Times. More at