Lake Pueblo is slowly filling with sediment that has reduced its capacity to hold water by about 7 percent over the last 40 years.
The equivalent of 19 feet of dirt over a football field, or 19 acre-feet, is coating various parts of the bottom of the reservoir, a natural consequence for any lake fed by streams and rivers.
The capacity for conservation storage — accounts that can be emptied and refilled — is down to 245,800 acre-feet.
The Bureau of Reclamation made the determination to apply the new limits at the beginning of the water year on Oct. 1 based on data collected in 2012, said Roy Vaughan, manager of the Fryingpan- Arkansas Project. It’s the first detailed look at sedimentation since 1994, when Reclamation found deposits were less than expected because the Arkansas River maintained its current at the bottom of the lake.
At Thursday’s meeting of the Southeastern Colorado Water Conservancy District board, the impact on future storage was discussed.
“We’re looking at water for the next generation,” said Jim Broderick, executive director of the Southeastern district. “We’ve been in a wetter period for the last couple of years, and reservoir levels have been near the top.”
About 25,000 acre-feet — nearly the amount Pueblo Water pumps in a year — could spill next spring if weather conditions are normal through the winter months and water is used in the same fashion as in the past. Lake Pueblo water levels still are about 138 percent of average, even though some water has been released over the past three months.
“What are the solutions?” Broderick asked Vaughan.
“Enlargement or dredging,” Vaughan replied quickly. “It’s been a 7 percent reduction over (40) years. That’s not to say something could be put in place. But what are the costs and who’s willing to pay?”
A third option would be to time storage and releases among users of the dam.
Two of the options, enlargement and re-operations, were considered in the district’s Preferred Storage Options Plan, largely abandoned when it stalemated after a decade of contention among Arkansas Valley water users.
Re-operations have largely been addressed by long-term federal contracts that overlay the basic protocol for Pueblo Dam’s operation.
Physical enlargement of the dam likely would mean reopening negotiations.
Dredging has its own issue. For one thing, the sediment is broadly spread over the floor of the lake, and is not lying in a big chunk that could be scooped out. According to the Reclamation report, it’s not settling in the area immediately above the lowest outlet on the dam.
Dredging might also worsen water quality, adding costs for treatment.
There are other economic considerations.
“The Fry-Ark water will stay in place because it’s cheap,” Broderick said. “But can you get your water out if you bring it in from transmountain sources? How much is the water worth? If we lose storage, how do we replace that?”
Board member Vera Ortegon said water users have managed water in the past so it does not spill. Water does not actually shlosh out of the dam, but is released to keep levels low enough to contain potential floods from upstream.
“We have not spilled much, have we?” Ortegon asked.
“No,” Vaughan said. “But we use additional storage in wet years, and then it’s pulled down in a dry cycle. You have to figure out what to do in wet years, so enlargement still comes into play.”