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Cost of increasing water storage on Verde $1B

Pictured is Bartlett Dam, which is located about 48 miles northeast of Phoenix. Salt River Project, the Bureau of Reclamation and 20 cities around the Valley have joined forces to figure out how to increase water storage on the Verde River, and one proposal is to raise the height of the dam. PHOTO COURTESY U.S. BUREAU OF RECLAMATION

Cities across the Valley have partnered with the Salt River Project to figure out how best to increase water storage on the Verde River.

In the long run, the partners want to offset the declining capacity of Horseshoe Reservoir by raising the height of the Bartlett Dam, which sits about 20 miles downstream, to store that amount of water and then some. The SRP and 20 partners are working with the federal Bureau of Reclamation on the effort.

The partnership is kicking off a four-year study into the feasibility of modifying the Bartlett Dam. Congress authorized the study last year, which followed a two-year long SRP and Reclamation appraisal study.

Modifying the dam

Ron Klawitter, a senior principal at SRP, said accumulating sediment at Horseshoe Reservoir has led to the loss of more than 45,749 acre-feet of water storage, about one-third of the reservoir’s original capacity. An acre foot contains 325,851 gallons.

Exploring how to address that loss, the feasibility study will look at the impacts of raising the height of the 287-foot Bartlett Dam by between 62 and 97 feet.

If the dam were raised by 97 feet, that would increase its capacity to 628,000-acre feet from its current 178,186-acre feet.

The increase would provide enough room to store the existing and restored water from Horseshoe Reservoir and the current amount of water stored in the Bartlett Reservoir. On top of that, it would add 305,784-acre feet of additional capacity to the 80-year-old dam.

During the appraisal study, Klawitter said they found that a 62-foot or 97-foot modification has about the same impacts on habitats and species in the area. While Klawitter said that makes the 97-foot modification more attractive because Central Arizona has such a need for renewable water surface supplies, he noted that the feasibility study would take a closer look at whether that is doable.

“There may be reasons we can’t build it quite that tall, so in that case, we would go down from there,” Klawitter said.

Long-term solutions

Klawitter leads SRP’s water system projects, which primarily focus on the long-term future of SRP’s service — looking at how climate change will affect its watersheds and reservoirs and what operational or infrastructural upgrades or changes are needed to support the system for the next century. SRP’s water service area is about 330 square miles in metro Phoenix.

Climate change is a consideration when it comes to modifying the dam, Klawitter said. The Salt River and Verde River have historically been very arid and warm, so the anticipated effects are different than those on the Colorado River, where rising temperatures have led to snowmelt and evaporation happening earlier in the season, heavily affecting the total volume of water in its reservoir system.

Instead, Klawitter said the Salt and Verde rivers’ total volume of water in the future will likely end up being pretty similar each year, but it may arrive in a much shorter period. That means having the capacity to handle bigger floods is important.

“We don’t expect to see less precipitation,” Klawitter said. “What we expect is there might be more flashiness in the events — so we might see bigger rainstorms, quicker runoff and melt periods, but in total, the decline we expect over the next 100 years is only about 2 to 2.5% precipitation-wise or runoff-wise.”

While Klawitter said the current situation at Horseshoe Dam isn’t an emergency, he noted that efforts like increasing the Bartlett Dam’s capacity are very long-term projects. If the study shows modification is feasible and Congress then authorizes construction, best-case scenario, the Bartlett Dam’s modification would be completed between 2030 and 2035.

“We have to be making a conscious effort to make progress each day to make sure we don’t find ourselves in an emergency situation where we don’t have the facilities or the operational plans necessary to manage the supplies,” Klawitter said.

Shown is a rendering of the proposed modification of Bartlett Dam to increase water storage. The blue lines represent the new height the dam would be raised to. RENDERING COURTESY SALT RIVER PROJECT

Partners share the cost 

More than 80 stakeholder groups — environmental groups, municipal water users, agricultural users, tribal bodies, land management and resource management agencies and others — participated in conversations during the appraisal study over the past two years.

Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University, participated in two of the focus groups during that time and said she thought the process was well done and well-staffed. She said the Valley was likely able to wait out the longer timeline for the Bartlett Dam modification.

“The cities that are impacted by the Colorado River shortage have plans for addressing potential shortage,” Porter said. “So, they either have other water supplies they can use, or in addition, they’ve stored up water supplies for use in the case of a shortage.”

The anticipated cost of the study is $10 million. Half of that will be covered by the federal government, with the other half split among SRP and the other partners.

One of the partners sharing in the cost is the town of Gilbert. Over the next four years, Gilbert is contributing an estimated $30,000 a year to the cost of the study. Salt and Verde rivers supplies account for about 30% of the town’s water portfolio, according to Gilbert Water Resources Manager Lauren Hixson.

“The resiliency of that supply is extremely important to us as part of our existing supply,” Hixson said. “The sedimentation of Horseshoe Dam is not an issue today but looking forward could be an issue for that supply reliability.”

In addition to preserving the existing supply, Hixson said the potential for adding additional renewable water supply is attractive as well.

“As a project participant, that’s something that we are interested in — in accessing a new long- term water supply,” Hixson said. “That adds diversity and resiliency to our overall portfolio.”

The sediment challenge

The Horseshoe Dam is unique. It’s an earthen dam, unlike the other concrete dams in the Verde and Salt River systems. It was built quickly during World War II to aid copper mining needed to support the war effort.

Moving the sediment out of Horseshoe Reservoir would be difficult and expensive. For one thing, it would require transporting the earthmoving equipment on a windy road to Bartlett and about 10 to 15 miles of rough, unpaved road to get to Horseshoe Dam itself. For another, ridding the reservoir of nearly 46,000-acre feet of sediment would create a 50-foot-tall sediment pile that covers 2.5 square miles. That pile would have to be managed in perpetuity to ensure it didn’t slide back into the water.

And that solution would be temporary, as sediment would continue to collect. Horseshoe Dam collects so much sediment in part because it’s at an area of the river that is wide and shallow. So, the river slows down and the sediment “really has an opportunity to settle out,” Klawitter said.

Klawitter said sediment accumulations aren’t that different on the Verde River compared to the Salt River, but because the storage system is smaller on the Verde, each acre foot of sediment accrued “directly impacts our ability to store water, and so we ended up spilling more water.”

“Removing the sediment is about a billion dollars; building the 97-foot modification of Bartlett is about a billion dollars,” Klawitter said. “With the modification of Bartlett, we get an additional 300,000 acre feet of capacity to benefit the wider region in Central Arizona.”

The Salt River system has four reservoirs and a total of about 2-million-acre feet in storage. In contrast, the Verde system has only about 276,000-acre feet, despite having about the same amount of run-off.

“What that means is during those big wet years, we’re able to capture as much water as we can on the Salt system, but in the same very wet years what we end up doing is spilling a lot of water on the Verde,” Klawitter said.

If Bartlett’s capacity is expanded, Horseshoe Dam will stay in place, though it won’t be used for water storage, as that capacity will be added to Bartlett. Instead, Horseshoe would be used to capture sediment and later pass it during large floods, to manage floods by slowing them in Horseshoe before passing them through the whole system and to restore and improve the habitat for various species that inhabit the area. For example, Horseshoe Dam acts as a barrier between more aggressive sport fish that are stocked in Bartlett and the native fish that live upstream.


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