The city of Aspen\ Credit: Brent Gardner-Smith/Aspen Journalism

ASPEN – Consultants for the city have lowered the estimate of how much electricity Aspen would likely produce if it built and operated a new hydro plant on Castle Creek and continued to operate the existing hydro plant on Maroon Creek, according to a new hydrologic report.

Instead of 6.2 million kilowatt hours a year as stated since last year by the city, the net annual amount produced by the two plants is now estimated to be 5.4 million kilowatt hours at full production and 3.1 million during the proposed “slow start” period. The new, lower estimates are a result of a more comprehensive look at available water against existing water priorities.

“We will update the financials accordingly,” David Hornbacher, the city’s director of utilities and environmental initiatives said Friday about the new power production estimate. “I would anticipate that it will slightly affect the number of years to pay back. But it will also still show a very positive economic impact over the project’s lifetime.”

In the report released April 4, hydrologists from Grand River Consulting have also clarified how much water Aspen proposes to divert from Castle and Maroon creeks to meet its various municipal, irrigation, hydropower and environmental goals.

Calculations based on the report reveal that from August through April each year, the city intends to divert at least 48 percent of the water from Castle Creek.

Matt Rice, the director of conservation in Colorado for American Rivers, said the organization has embraced new ecological studies that indicate taking more than 20 percent of a river’s water causes long-term environmental degradation.

Castle Creek on April 9, 2012, flowing near the Marolt housing project.Smith Credit: Brent Gardner-Smith

With a proposed new hydro plant running at full production, the city intends to divert a monthly average of between 12.6 cubic feet per second and 40 cubic feet per second (cfs) of water from Castle Creek to meet its municipal, irrigation and hydro needs.

That would leave 375 cfs in Castle Creek in June during high runoff, but only 13.3 cfs in the stream each year from January through May.

In June, the city would be diverting only 10 percent of the peak runoff, but in January the city would be diverting 64 percent of the stream to meet its municipal and hydro needs.

From Maroon Creek, the city expects to divert between 9.8 cfs and 34.5 cfs of water, leaving 342 cfs in June but only 14 cfs of water during the seven months from October through April.

That’s diverting between 9 percent and 66 percent of the water from Maroon Creek at the point of the city’s diversion dam, just above the T Lazy 7 Ranch.

“They are taking everything they can right down to the minimum streamflow,” said Ken Neubecker of the Western Rivers Institute. “It is going to effect the health of the stream’s riparian system.”

Neubecker developed the depletion percentages using data from the city’s report.

The city has previously released figures indicating that at full production, the hydro plant would take a maximum of 39 percent of water from Castle Creek and 29 percent from Maroon Creek during winter months.

However, those figures apparently did not take into account the city’s municipal and irrigation diversions, and the information in the new report is a conservative estimate of how much water will be in the streams at the city’s diversion points.

Water impounded behind a dam on Maroon Creek entering a diversion structure on Monday, April 9. On this day, about 9 cfs of water was being taken out of the stream and sent to the existing Maroon Creek hydro power plant about a mile-and-a-half downstream. At higher flows, over 50 cfs of water is diverted toward the plant.Smith Credit: Brent Gardner-Smith

The report was prepared by Grand River for the city and submitted on April 4 to the Federal Energy Regulatory Commission. The city is seeking approval from the federal agency for a new hydro plant on Power Plant Road under the Castle Creek highway bridge.

The report estimates how much water in both streams would likely be available by analyzing streamflow data collected on Castle and Maroon creeks from 1970 through 1994, when gauges on both streams were then taken off line by the U.S. Geological Survey, which operated them.

The 25-year period included one of the driest years in the last 100 years — 1977 — and one of the wettest years in 1984.

By the time Castle Creek reaches the city’s diversion dam, about 3 miles above the stream’s confluence with the Roaring Fork River, only about 2 cfs has been diverted and consumed by upstream users.

And so in an average water year, there is an average of 415 cfs available in June, but only 25.9 cfs in April.

On Maroon Creek, about 9 cfs of water is taken out of the river and consumed by the Herrick Ditch before the stream reaches the city’s diversion dam.

The resulting monthly average flow in June on Maroon Creek at the city’s diversion point is 376 cfs. In March, it is 24 cfs.

Next, hydrologists at Grand River Consulting looked at the city’s water priorities.

The highest priority is meeting municipal water demands, for which the city diverts between 7.3 cfs and 7.9 cfs from Castle Creek and essentially nothing from Maroon Creek.

The city’s actual use requires between 2.8 cfs and 3.2 cfs of water from Castle Creek, but the city diverts a steady base flow of 4.5 cfs to fill the Thomas Reservoir at the city’s water treatment plant.

The small reservoir serves as a buffer against large instantaneous demands in the city, such as in the case of a fire. Unused water is returned to Castle Creek via an existing drain line running under Castle Creek Road.

The city’s second priority is to meet minimum streamflows designed to protect the environment.

On Castle Creek, the city has committed to leaving at least 13.3 cfs in the stream, both below its primary diversion point and below its three irrigation ditches farther downstream.

The state has set a minimum streamflow of 12 cfs for Castle Creek, but the city has embraced 13.3 cfs upon the recommendation of its biological consultant.

On Maroon Creek, the city has committed to leaving at least 14 cfs in the stream, which is consistent with the state’s minimum level.

The third priority for the city is to divert water from Castle Creek for irrigation into the Si Johnson Ditch, the Holden Ditch and the Marolt Ditch. This requires between 4.8 cfs and 15 cfs of water from May through October.

One of the city\ Credit: Brent Gardner-Smith/Aspen Journalism

Next, the city intends to divert at least 10 cfs of water to the existing Maroon Creek hydro plant.

After that level, the city intends to divert more water from Maroon Creek into a pipe leading to Thomas Reservoir and down a penstock to the new Castle Creek hydro plant.

But the report notes that due to the “hydraulic properties of the Maroon Creek pipeline … when water is diverted to Thomas Reservoir and to the Castle Creek Energy Center, the effective capacity of the pipeline to the Maroon Creek hydroelectric plant is decreased.”

“You’ve got a push and pull there,” said Kerry Sundeen, a hydrologist and president of Grand River Consulting. “The more you put through Castle Creek the less you put through Maroon Creek.”

Today the Maroon Creek hydro plant generates about 1.9 million kilowatt hours of electricity a year using between 9.8 cfs and 59 cfs.

The report estimates that in the future, without the new Castle Creek plant, there would be enough water in Maroon Creek to produce an average of 2.2 million kilowatt hours of power a year.

But with a new Castle Creek plant running at full production, only 730,000 kilowatt hours a year would be produced at the existing Maroon Creek plant.

The new Castle Creek plant at full production is expected to generate 6.8 million kilowatt hours a year.

But subtract the 1.4 million hours of power no longer being produced at the Maroon Creek plant, and the net power production of the two plants drops to 5.4 million kilowatt hours a year.

Last year, the city published a primer on the project that says 6.2 million kilowatt hours would be produced annually after installation of the new Castle Creek plant.

While the primer does not explain the concept of “net” power production between the two hydro plants, Hornbacher, the city’s utilities director, confirmed Friday that the 6.2 million kilowatt figure used was a net number.

About 9 cfs of water flowing out of the Maroon Creek hydro plant on April 9, 2012.Smith Credit: Brent Gardner-Smith

The report also details how much water would be diverted, and how much power would be produced, under the proposed “slow start” plan.

That plan is designed to initially leave more than the state’s minimum streamflow in both creeks and then monitor the health of the streams to see if more water can eventually be diverted.

Under the slow start scenario, the city would leave — in an average water year — between 18 cfs and 24 cfs in Castle Creek from December through April, as compared to leaving a consistent 13.3 cfs during the same period.

The slow start scenario does not significantly change the amount of water the city would leave in the stream below its Maroon Creek diversion point, however, as there would still be seven straight months when the streamflow was maintained at 14 cfs.

The slow start scenario does, however, significantly lower the amount of net power produced using water from both streams to power both hydro plants.

Instead of a net of 5.4 million kilowatt hours during full production, the slow start produces an average of 3.1 million kilowatt hours.

Editor’s note: This story was done in collaboration with the Aspen Daily News and was published in the newspaper’s Monday, April 9, edition. After publication, city officials said there were several errors in the report and that they were revising the report to reflect more accurate estimates of how much water was available and how much power would be produced. As of late Monday evening, when the story was posted on Aspen Journalism, city officials said they thought the amount of net power produced by the new Castle Creek plant and the existing Maroon Creek plant would be 6.1 million kilowatt hours, not 5.4 million kilowatt hours as stated in the report. We are posting the story here, as it ran in the Daily News, for the record, with the expectation that a follow-up story will be published after city officials have corrected their report.

Brent Gardner-Smith

Brent Gardner-Smith, the founder of Aspen Journalism, and who served as AJ’s executive director until August 2021 and as editor from 2011-2020, is the news director at Aspen Public Radio. He's also been...