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Deep Dive: How many people can the Treasure Valley’s water supply support? It’s complicated.


As more and more people move to the Treasure Valley and climate change drives summer temperatures higher, water is top of mind. 

Is there enough of it to sustain our rapidly growing metro area decades into the future? Does the Treasure Valley have a maximum number of people our water supply can support? And how will climate change impact the whole system?

There are no easy, clear-cut answers to these questions. 

The Treasure Valley is unique in the Mountain West due to its rich water supply coming in from multiple sources. The combination of abundant groundwater, the network of dams storing water in the Boise foothills and the proximity of the Snake and Boise rivers give Southwest Idaho a lot of room to add population into the future. But, our policies for managing the water we have will determine how far the region’s water supply can stretch. 

‘It’s up to us’

The Treasure Valley’s water comes from two different, but distantly related, systems, each with their own long-term questions for sustainability. 

First, there’s the network of irrigation canals stretching from the Boise Foothills clear past the Oregon border watering acres and acres of agricultural fields. This system, called surface water, is fed by the network of three dams in the mountains storing and collecting rainwater to be released down into the valley to be used. The majority of this water is for farming, or to a lesser extent, for watering lawns and other urban areas. 

The second source of our water also comes from the mountains but at a much deeper level. Most drinking water that comes out of your tap from Veolia, the private water company formerly known as Suez, or other municipal providers comes from a network of aquifers deep in the ground, hundreds of feet below the surface. Hydrologists are still studying the source and underground flow of this water, but they theorize it comes from snow melting high in the Sawtooth Mountains, leaking into the ground through tiny cracks in the earth and making its way to the Treasure Valley over the course of hundreds of years. 

Lejo Flores, an associate professor of hydrology at Boise State University, said the Treasure Valley’s water supply doesn’t put a hard limit on how many people could live here in the future, but eventually, our area could see watering restrictions and other requirements to preserve common in the Denver metro area or parts of California. Ultimately though, he said our water supply is going to depend on if we continue to build sprawling subdivisions with large, water-guzzling lawns, or move toward more dense development. 

An aerial view of homes at Locust Grove Rd and Ustick Rd. in Meridian Photo: Charles Knowles/Shutterstock

“The fundamental question of how many people can sustainably live here from a water perspective is, like a lot of things, up to us…” Flores told BoiseDev in an interview. “Do we get a continuation of track housing and folks wanting to have HOAs that enforce green laws or do we see more densification? Or do we see both? And how do we manage that?”

The Idaho Legislature acted earlier this year to give the final approvals for a project raising Anderson Ranch Dam six feet, adding 29,000 acre feet of storage per year to the system after up to four years of construction and over $80 million dollars. But, just because the dam is taller and can hold more water, doesn’t mean there will be enough snowmelt each year to completely fill it. Estimates suggest this higher dam will only fill with an additional 16,000 acre feet per year. 

“Over a span of like ten years, the amount of water stored every year in its total capacity goes up, but if you can ever reach that total capacity is a question for the climate,” Flores said. “The climate controls that. Adding capacity, at the end of the day, on the surface water system may have some benefits in wetter years, but the question I have is do we really have issues in wetter years?”

Water demand projected to double by 2065

The Boise metro area is growing, and so will the demand for water. 

In 2015, the Idaho Department of Water Resources hired hydrologist Christian Petrich to complete a study projecting the demand for water in the Treasure Valley for the next fifty years. At the time, domestic, commercial, municipal and industrial users (basically everyone except agriculture operations) was using 110,000 acre feet of water per year. 

Petrich estimated over the next 50 years the metro area would boom to 1.57 million people and demand between 219,000 and 298,000 acre feet of water per year, at least double the 2015 demand. And on top of this, he estimates average temperatures will increase between 1.9 and 6.1 degrees in this period, boosting water evaporation from 5% to as much as 20%. 

These projects also build in an assumption that water conservation methods are added to the region over time, reducing usage from between 10% and 30%. And even with these efficiency steps, his best estimate would be an additional demand for 165,000 more acre feet of water per year by 2065. 

Another factor baked into this analysis was an assumption that the population of Elmore County will continue to decrease to 22,400 residents by 2065, but this could change due to rapidly growing housing costs in Ada and Canyon counties in the years since Petrich completed this analysis. 

Spacial distribution of Treasure Valley water demand by 2065. Courtesy of HDR SPF

Petrich returned with a new report on the state of the Treasure Valley’s water supply earlier this year, but this time he was hired by Ada County’s largest water provider. 

Veolia hired Petrich to compile a report analyzing different options for how the region can supply these additional 165,000 acre feet of water for the next 50 years. His report found there’s enough water to support the growing need for water, but they will require cooperation across multiple entities throughout the region to put the infrastructure in place. 

This report examined the feasibility of several options, including additional storage up in the dam system in the foothills above the city, additional groundwater pumping, pulling water from the Snake River and water recycling. 

Small steps can go a long way 

Petrich’s report says the Treasure Valley can inch towards its water goal with several incremental changes to increase the supply, even if these policies won’t get the valley all the way to the amount needed by 2065. 

The first source the area can use to increase water supplies is in the ground. Despite all of the pumping for groundwater going on across the valley, groundwater levels have remained stable in most of the Treasure Valley and can support more pumping to help meet the region’s goals. Over time, groundwater levels have risen in many parts of the valley as shallow aquifers have been recharged by agriculture and leakage from the bottom of irrigation canals. 

But, as development has overtaken agricultural land, particularly in Southwest Boise, some of these shallow aquifers are dropping and pushing residents to either dig deeper wells or try to coordinate with their neighbors to hook into Veolia’s municipal water system. This aquifer network close to the surface is separate from the deeper, stable water system Veolia and other municipal providers are pumping from to supply the region. 

The stable water levels and return flows making their way to the Boise River suggest the region can increase pumping, but this can’t be the sole answer to our water needs. Some areas, like the Boise Foothills, the area near Micron and the south and east portions of Ada County have limited water providers can pull out of the ground. Other areas of the aquifer cannot be pumped due to water quality concerns or if the water exceeds 85 degrees.

The report said said another roughly 15,000 to 20,000 acre feet per year could also come from reusing treated wastewater for irrigation of parks, golf courses, other wide open green spaces. Petrich wrote that reuse can be expensive, but the most cost-effective way to utilize it is to incorporate it into areas already growing instead of trying to retrofit already built out areas with a new water delivery system. 

Reusing treated wastewater is currently a major part of the City of Boise’s playbook to combat climate change and the drought that comes with it. But, despite Veolia’s support for recycled water nationwide and their stamp of approval on this report suggesting it as an option for irrigation, the company has been deeply skeptical of Boise’s plans to treat industrial wastewater and recharge aquifer south and east of Boise with it. 

Using extra water in years with flooding could give the Treasure Valley a boost, the report found, but it’s unpredictable when this extra water would be available for short periods of time. Plus, additional planned projects to raise dams in the Boise Foothills to add more water storage would decrease how much floodwater is available. 

The Treasure Valley’s ‘leaky bucket’ 

In many ways, Idaho’s system of water laws is similar to other western states where water is scarce. 

The system of water rights is largely based on who has held them the longest, with the oldest water right holders getting access to the water later in the irrigation season while more junior holders get cut off early. But, the way Idaho handles water has one key difference from states like Colorado: Water rights aren’t typically sold and transferred when a property changes from farmland to a subdivision. Instead, those water rights often stay with the parcel and it leaves excess water not used for lawn irrigation to go unused. 

Jeff Fereday, a former partner at Givens Pursley and longtime water lawyer, argues this tendency to let water rights stay with parcels after areas urbanize violates Idaho law requiring water rights to be for “beneficial uses.” He said this mindset likely comes from Idaho’s long history of landholders wanting to hold onto their water rights against all odds, even when transferring water rights makes sense in areas like the Treasure Valley where water is being used in new and different ways than before.

Fereday said Idaho’s policy of allowing acres of subdivisions and shopping centers to keep water rights for hundreds of acres more of land than they will use on their lawns effectively creates a leaky bucket, with more usable water flowing away to the Pacific Ocean every year. 

“In other words, this bucket that was full in 1930, but when we started paving over farmland we put a hole in the bucket and in Colorado they say ‘that water needs to go somewhere else so let’s attach a pipe and put it over here’,” he said. “In Idaho we don’t do that. We just let the bucket run to nobody’s benefit. But the problem with galloping developing is that hole is getting larger every year.”

A Treasure Valley irrigation canal. Photo: Don Day/BoiseDev

And watering lawns isn’t the same as watering fields of crops. While farmers slowly water portions of their fields over a number of days and have breaks for harvest time, homeowners often have to water their grass daily throughout the warmer months of the year to keep it green. Turf grass is also one of the most water-intensive plants grown in the United States, outpacing the water needs of all but only a few crops. 

So, even though the areas covered by lawns and other urban irrigation are much smaller than the sprawling fields that once grew crops, Fereday says studies conducted by hydrologists show subdivisions consume more water per square foot than agricultural land. 

“But (studies from hydrologists) said (subdivisions use) 20% less, but 20% is a lot of water,” Fereday said. “And furthermore those studies only looked at subdivisions. They didn’t look at commercial property. Just drive to Nampa and look around. It’s unbelievable what’s out there and most of it is zero or near zero irrigation.”

Brian Patton, the executive officer for the Idaho Water Resources Board, told BoiseDev that hundreds of water rights are transferred every year in Idaho, including from one location to another. However, many large water rights in Idaho are appropriated to irrigation districts, which have to use them in a specific service area. As places urbanize, the water can be used for different uses, but the water rights are still connected to that one area.

“In both cases, (irrigation districts and canal companies) are governed by a board elected by the users of the water delivery system,” he wrote in an email. “Any transfers of water rights owned by these organizations would have to be undertaken by their governing boards, and follow the set of laws that govern these organizations – including disposition of public property in the case of an irrigation district.”

After publication of this story, Fereday said Patton’s comment illuminates his point. He says the state and irrigation districts should be doing more to ensure water we already have isn’t being wasted and enforce their own laws around beneficial use.

“The point underlying Patton’s comment is that yes, of course the irrigation districts and canal companies, who divert the overwhelming bulk of Boise River water, could market it, could transfer it, could make it available for other uses and future growth (just like their brethren do in other states), but they simply decline to do so!” he wrote in an email. “And for reasons that have never been explained, much less justified.

“What are we, as a state, supposed to make of this, especially when the water resource belongs to the people and water right transfers are necessary for our future?”

Go big, or go dry

Veolia’s report says more groundwater pumping, reusing wastewater and the hope of reappropriating water from agricultural land won’t be enough to cover the estimated population growth for the next 50 years. In order to get there, Petrich’s report for Veolia maps out a range of much more ambitious projects required to harness the region’s water into usable drinking water. 

The Snake River is the Treasure Valley’s biggest bet for meeting its water needs in the decades to come, which Petrich’s report said could be configured in a variety of ways. The water could be pumped out of the river to a treatment plant in Kuna or the south end of Boise and used for drinking water or it could be pumped to Lake Lowell, allowing water pulled from the Boise River to be used for other purposes instead of refilling the reservoir. 

Snake River water could also be used for agriculture, replacing groundwater pumped by farmers that could instead be used for drinking water. The water could also be treated and pumped into the ground to recharge the aquifer in the area. These options are all expensive and would require heavy collaboration from United States Bureau of Reclamation, the Boise Project Board of Control that oversees the New York Canal and other irrigation districts, but the water supply is “the most certain” potential supply for municipal water. 

More water could also be pumped out of the Boise River, but only areas west of Star have additional room for more water rights. This portion of the river is heavily supplied by what’s known as return flows, or water that has passed through the irrigation system, spilled back into drains and reentered the river. This gives the river in the western end of the Treasure Valley a lot of supply, but this could change if substantially more groundwater is pumped from the valley or if other factors change the water system. 

This additional water would also require costly treatment facilities just like the Snake, driving up the price tag for these options despite the high water levels. 

Correction: The original version of this article misstated some of the claims Jeff Fereday made about Idaho’s current system of transferring water rights. It has been clarified to reflect his views more accurately.

Margaret Carmel - BoiseDev Sr. Reporter
Margaret Carmel - BoiseDev Sr. Reporter
Margaret Carmel is a BoiseDev reporter focused on the City of Boise, housing, homelessness and growth. Contact her at [email protected] or by phone at (757)705-8066.

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