VALLECITO – Vallecito Dam is due for some serious upkeep.
Built in the late 1930s, the infrastructure that maintains Vallecito Reservoir is decades beyond its original design life. So far, things have held up remarkably well, and the dam’s engineering remains impressively sound.
But aging materials and erosion have caused significant damage to the dam’s emergency support structures, and a major repair project is coming down the pipeline sometime in the next several years.
“We’ve got this issue and we know it’s here. It hasn’t been clandestine; we’ve told people about it forever,” said Ken Beck, superintendent of the Pine River Irrigation District. “But it’s a nail-biter for a superintendent and dam tender.”
PRID operates, maintains and manages Vallecito Dam and Reservoir, which holds and delivers supplemental irrigation water to 65,000 acres of land downstream – the lifeblood for ranchers and farmers who hold water rights with the district.
The repair project – about which little has been decided beyond the fact that it must happen – will be a massive undertaking. Beck estimated it could take roughly two to four years to complete once ground is broken, likely changing some of the regular operations of the reservoir.
There is the potential that irrigators, ranchers and farmers who rely on consistent water deliveries would feel some impact – but Beck said how much and if at all is dependent on a variety of factors, like the weather and the time of year when the construction is done.
“They’ll try and stage that work so that they’re having the least amount of impact that they can on the farmers or on the irrigators,” he said.
The project is also important from the standpoint of the Southern Ute Indian Tribe, which is entitled to one-sixth of the reservoir’s total storage capacity. That water is used primarily for tribal agriculture and water management.
“Vallecito Reservoir is essential to the well-being of our people and the protection of our homelands,” said Chairman Melvin Baker of the Southern Ute Indian Tribe.
Given the importance of the reservoir to both the tribe and the surrounding communities, “timely repairs are essential,” he said.
The primary issue is the dam’s upper spillway, a critical safety structure designed to release water during extreme runoff or flood events.
Vallecito’s upper spillway includes three radial gates and a concrete chute that carries water to be released downstream safely without damaging the dam itself. Any damage to that infrastructure is a critical issue, and can compromise the dam’s ability to manage high water and protect downstream communities.
In 2017, PRID conducted a dye test to assess the spillway’s integrity, Beck said. Dye placed upstream later appeared in areas downstream where it should not have surfaced if the structure were intact, confirming that water was migrating beneath the concrete spillway.
That process – known as “piping” – can carry sediment out from under the structure and weaken its foundation.
After the dye test, the Bureau of Reclamation launched a series of investigations that revealed large underground voids – some as large as 4 by 10 feet – beneath portions of the spillway.
Beck said it was determined the upper spillway is unsafe to use except in dire emergencies, because uncontrolled flows could accelerate erosion and threaten the dam’s integrity.
“Now, we get into a flood, then we’re going to use that spillway, regardless,” he said.
But to mitigate against the worst-case scenario, operations have since been modified to keep reservoir levels 1 foot below the radial gates at all times.
While the spillway does not pose an imminent threat to the dam or downstream safety, each year it goes unaddressed is taking a gamble on the district’s luck, Beck said.
A large flood combined with high reservoir levels could create a serious emergency situation, he said. The recent flooding at Vallecito underscored the urgency of making repairs sooner rather than later.
The reservoir narrowly avoided serious flooding during last fall’s storm as flows on Vallecito Creek tributaries surged to more than 10,000 cubic feet per second. The situation was manageable largely because the event occurred late in the year, after irrigation season when the reservoir was relatively empty, Beck said.
If the storm had hit in the summer when the reservoir is typically near capacity, it could have been a much different and much more dangerous situation, he said.
Mike Canterbury, dam manager for the irrigation district, pointed to the 2017 Oroville Dam spillway failure in Northern California as a cautionary example.
After historic rainfall forced operators to release high volumes of water, cracks beneath Oroville’s spillway widened, concrete slabs lifted and failed, and erosion rapidly worsened. Roughly 200,000 people were evacuated, and damages reached into the hundreds of millions.
In September, PRID and Bureau of Reclamation staff members met to brainstorm long-term solutions and evaluated numerous options – from various spillway configurations to full decommissioning – as part of a standard, comprehensive review process.
At this stage, however, Beck said, only two alternatives are actively being considered.
The first would replace the existing spillway “in kind,” and maintain its current alignment and design. The second would also involve replacing the spillway, but would remove the existing curve and put in a straight spillway.
Regardless of the final plan, an additional emergency spillway would likely need to be constructed, Canterbury said.
It would help continue to send water to irrigators – a primary focus of the Bureau of Reclamation.
However, Beck cautioned that some level of disruption is possible, and irrigators may need to prepare for one or more seasons with reduced water availability.
Long-term repairs could require the reservoir to remain partially empty for several years, depending on the multitude of factors, both controllable and uncontrollable.
Beck said the Vallecito project is competing with hundreds of similar aging infrastructure concerns across the West.
“There are a lot of structures just like this that have needed attention for years and years,” he said. “They’ve done things to try to move this project up the list, and it’s getting there – and that’s a positive thing.”
Budget cuts and staffing losses at the Bureau of Reclamation have also slowed timelines, officials said. But they emphasized the deliberate pace reflects the complexity of repairing aging dams without creating unintended consequences.
Beck said the working relationship between PRID and the Bureau of Reclamation is as strong as it has ever been.
“There’s a lot of trust there and a lot of respect, as there should be,” he said. “That’s something we’ve really worked at.”
While no final design has been selected, officials estimate repairs would cost hundreds of millions of dollars – potentially more than $500 million – and take several years to complete.
While no final design has been selected, officials estimate repairs would cost hundreds of millions of dollars – potentially more than $500 million – and take several years to complete.
Most of the cost would fall to the federal government, Beck said.
Years ago, PRID had the opportunity to purchase not only the water rights but also the dam itself. The district opted not to buy the structure – a decision Canterbury called one of the smartest PRID ever made.
As a result, the district is responsible for about 15% of the repair costs rather than the full amount.
“That’s still a lot of money,” Beck said. “So we get a little bit out of sorts when somebody comes up and says, ‘We’re not going to pay a recreation fee.’”
jbowman@durangoherald.com
