Spruce and aspen trees growing on the dam on the east side of the Thomas Reservoir in Aspen. The pond-size reservoir is on the hill behind the Castle Ridge apartment complex and Aspen Valley Hospital.
Spruce and aspen trees growing on the dam on the east side of the Thomas Reservoir in Aspen. The pond-size reservoir is on the hill behind the Castle Ridge apartment complex and Aspen Valley Hospital. Credit: Chris Council / Aspen Daily News

A state dam safety engineer is requiring the city of Aspen to cut down 25 trees on the Thomas Reservoir dam and said the trees may already have caused “dam safety problems.”

“The sloughing and disturbance seen during the inspection around the large roots indicate the dam safety problems may already be starting,” John Blair, a dam safety engineer with the Colorado Division of Water Resources, told the city in an April 11 letter after an inspection on April 4. “As such, the dam was rated conditionally satisfactory and the trees need to be removed as a high priority.”

In a section of his inspector’s report labeled, “Items requiring action by owner to improve the safety of the dam,” Blair instructs the city to “remove all trees from the downstream slope of the east dam including their root systems. This will require significant excavation into the embankment and proper backfill compaction.”

The worry is that if one of the trees dies and falls over, or is blown over in a wind storm, its uprooting could compromise the earthen dam where the trees are growing.

David Hornbacher, the city’s director of utilities, said the municipal government will comply with the state’s directive and has filed for a permit from the city’s forestry department to remove the trees.

Sixteen Englemann spruce trees, ranging from 30 feet to 40 feet tall, stand on the east crest of the dam today, while nine aspen trees are growing behind the spruce trees on the lower face of the dam.

The spruce and aspen trees range from 9 inches to 14.8 inches in diameter, according to Hornbacher.

The city plans to have the trees cut down by September and hopes to have the tree roots excavated from the dam and filled back in by winter, according to Hornbacher.

The city also plans to retain a consultant to review how best to excavate the tree roots while maintaining the integrity of the dam.

“Should the results of this engineering review and plan recommend more immediate actions, the city will take further precautionary measures that may include drawing the reservoir down while accelerating the work to remove the hazard,” Hornbacher said in an email.

The new spillway on the east dam on Thomas Reservoir. The trees in the background will be cut down and the roots excavated.
The new spillway on the east dam on Thomas Reservoir. The trees in the background will be cut down and the roots excavated. Credit: Chris Council / Aspen Daily News

Thomas Reservoir is closer to a pond than a lake. It holds 10 acre-feet and has a surface area of 1 acre, according to the state engineer’s inspection form. By comparison, Ruedi Reservoir above Basalt can hold 102,000 acre-feet at peak fill and has a surface area of 997 acres.

The semicircular Thomas Reservoir dam is an earthen dam and is 280 feet long, 19 feet high, and 14 feet wide at its crest. It looks more like a large berm than it does a concrete or rock dam.

Blair’s April 4 review of the dam was the first official state inspection since 1989.

At that time, the reservoir held 4 acre-feet and Blair concluded that “since the dam creates no public safety hazard, we have removed it from our scheduled dam safety inspection program.”

But last year, after the city and the state conferred on the city’s plan to install a 42-inch pipeline running out of the reservoir, the state reasserted its jurisdiction over the dam and changed its classification.

The pipeline is intended to feed a hydroelectric power plant that would be 4,000 feet down the hill underneath the Castle Creek bridge, as well as serve as an emergency drainline for the reservoir. The city is currently seeking federal approval for the power plant.

The state changed the classification of the dam and reservoir from “no public hazard” to “significant hazard,” primarily because two public housing projects — Twin Ridge and Water Place — have been built below the reservoir since 1989.

From July to December 2011, crews from Western Summit Constructors worked to install an outlet structure and the 42-inch pipeline. On Feb. 27, the state reviewed and approved the work and gave the city the OK to refill the reservoir, which had been drained for construction.

But after his April 4 inspection, Blair sent his letter stressing the hazard posed by the trees growing near the dam, and said the state was now considering the dam to be rated at “conditional full storage.”

“Overall the dam and its appurtenant structures are performing satisfactorily after its first fill after the major modification took place last year,” Blair wrote, also noting the city’s “excellent monitoring program.”

“However the large trees on the downstream slope of the old east dam, which was not part of the modification, [are] a concern,” Blair wrote.

The “old east dam” is a manmade dam last modified in the mid-1960s to hold water back with the help of the natural slope. Last year’s construction on the north side of the reservoir resulted in what the state now refers to as the “new north dam.”

“The associated root systems penetrate into the [old east] dam,” Blair told the city. “When the vegetation dies, the decaying roots can provide paths for seepage that could lead to a possible piping failure of the dam.

“Also, if the trees were to fall over from high winds or other external means, the associated uprooted root balls would leave large holes in the dam effectively reducing the dam’s cross section and shortening potential seepage paths, which could also lead to a piping failure of the dam.”

Hornbacher said cutting down the trees and filling in the holes their roots will leave behind will eliminate the concern.

The engineer’s report also instructs the city to “monitor trees on the downstream slope visually for more disturbance, signs of seepage, signs of falling and tearing out portions of the dam, until they can be removed properly.”

The city's Thomas Reservoir holds 10.3 acre-feet of water. The new outtake structure for the city's emergency drain line/penstock is visible on the far side of the reservoir.
The city\ Credit: Chris Council / Aspen Daily News

Rob Covington, the raw water and hydroelectric supervisor for the city, responded to Blair via email on April 13.

“I will be moving forward on correcting the deficiencies in the report immediately,” Covington wrote. “The items that are still part of the construction contract will be corrected by Western Summit Constructors starting in May.

“The tree removal is going to be a substantial cost and will need to be budgeted for by the city of Aspen. I will get the permitting and submittal for the tree removal started right away and will stay in touch with you on the dates when the actual work will be performed.”

Convington’s email is the only official correspondence from the city in response to Blair’s letter that was in the files at the Division of Water Resources office in Glenwood Springs the week of May 21.

There were also 10 other items that Blair said needed “action by owner to improve the safety of the dam,” but they were more modest in scope than removing the trees.

They included such items as “provide grass erosion protection for the new north dam” and “extend toe drain outfall off of the embankment.”

Editor’s note: This story was developed in collaboration with the Aspen Daily News and was published in the paper on Friday, June 1, 2012.

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...