Gross Reservoir Expansion Project

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Why is Denver Water expanding Gross Reservoir?
Our current water supply system isn’t ready to handle the very real threat of future water shortages. Expanding Gross Reservoir will bolster water supply and reliability with more storage to meet the Denver Metro Area’s existing and future needs.
Is this project really needed?
Yes. The Gross Reservoir Expansion Project is important in protecting against potential catastrophic events such as fires, landslides, floods, drought and infrastructure failures. Shortfalls in our overall water storage system could occur as early as 2022 if we don’t increase storage capacity. Expanding Gross Reservoir provides that storage capacity, but the project will take several years and must begin before a crisis is upon us.
If a greater level of water conservation were required in the Denver Metro Area, could we postpone or not need to build this project?

No, conservation is critically important, but it doesn’t replace the need for this project. In 2014 our customers used less water per capita than they did in the last 40 years. Quite an accomplishment considering the Denver Metro Area population has increased by 350,000 people since 1970.

Our consumers embrace the need for water efficiency, but even when we’ve seen our customers reduce their usage by a third as they did in a previous drought year, we still run the risk of running out of water on the north end of our supply system – of which Gross Reservoir is a critical part – and this project helps offset that imbalance.

The fact is we’re vulnerable on several fronts. We are short of supply and storage capacity on the north end of our system making us vulnerable to drought. We’re also vulnerable to an infrastructure failure that could occur on the south end of our system. Consider if something like the August 2015 Animas River contamination occurred upstream on our South System; we’d be left with a limited capacity to serve the Denver Metro Area from our north system. It is not worth the risk.

When this project is completed, how much more of our winter runoff can we keep in Colorado during wet years?

It varies. Most runoff available to Denver Water during the wet winter of 2014 and spring of 2015 flowed out of state because existing Denver Water reservoirs were full and there was no place to capture and store it. Because the extra space is designed to capture and store water in average and wet years, if the Gross Reservoir Expansion Project had been completed in 2014, there would have been 72,000 acre-feet of new water storage space available by the summer of 2015 to collect and store some of that water for use during drier times.

Why was Gross Reservoir not built to its capacity originally?
When it was built, its capacity was sufficient to meet customer needs. Forethought and fiscal responsibility aligned to prepare for today’s needs, as decisions made then enable our work today to meet the challenge of an economically and environmentally responsible storage solution.
Why has this project taken so long?

This project requires federal permitting, which in itself is a lengthy, complex process. It was important to Denver Water that all impacts to the surrounding area and the water system were analyzed and that the most environmentally responsible and economical alternative was chosen to increase storage. In partnership with federal authorities, more than a 14 years of study has identified potential impacts and mitigation strategies. We are proud to have received federal approval from the U.S. Army Corps of Engineers in 2017 and final federal approval from the Federal Energy Regulatory Commission in 2020.

This is a great example that water projects are not a “just in time” enterprise – they must be carefully planned, undergo environmental studies, have very detailed designs, and be constructed in a safe manner to last for generations.

Who benefits from this project?

The primary beneficiaries of this project are the 1.5 million people Denver Water currently serves and many of the projected 8.1 million who will call Colorado home by 2050. The environment and West Slope stakeholders also benefit through two agreements related to expanding Gross Reservoir: the Colorado River Cooperative Agreement and the Mitigation and Enhancement Coordination Plan. In addition, Denver Water partnered with the Cities of Boulder and Lafayette to provide additional storage in Gross Reservoir in the form of an environmental pool that will allow water releases to South Boulder Creek to improve stream health.

Perhaps most importantly, Denver Water has made significant investments in emergency and disaster planning. Beyond supply reliability and environment safeguards, additional storage means greater water resources in times of emergency.

When will the project be complete?

Preliminary design has begun and will accelerate now that the Record of Decision was handed down from the U.S. Army Corps of Engineers on July 6, 2017. Final design is expected to be complete in 2021. That will allow construction to begin in 2022 with a construction completion anticipated in 2027 and, depending on water availability, it will take approximately five years to fill the reservoir to its new capacity.

What would happen if we didn’t do this project?

Denver Water has a responsibility to meet the needs of the 1.5 million people we serve today and future demand created by population growth. Without the project, Denver Water’s North System will remain vulnerable to catastrophic events and continue to be ill equipped to handle an increase in stress to the system. Currently, a single dry year or emergency — such as a forest fire or treatment plant shutdown — puts our water supply in jeopardy. Expanding Gross Reservoir helps us avoid running out of water in any given year and helps us put water where we need it.

What is the North System and why is it important?
Denver Water’s supply system is essentially in two parts: the North System and South System. During a span of several dry years, the North System, in its current state, will not have enough storage capacity to supply water to residents. Nearly ninety percent of Denver’s water supply storage rests in Denver Water’s South System, so the entire system is out of balance. This project will help to balance out the two systems and provide more water storage for the entire system, should the South System be shut down again as it was in 1996 following the Buffalo Creek Wildfire.
How much will this project cost? Who will pay for it?
When the reservoir begins to fill to its new, increased capacity in 2026, the project will have spanned nearly a quarter century from the start of environmental study and permitting to the end of construction. The total investment needed to finish the project, or the estimate at completion (EAC), is currently anticipated at about $464 million (in 2025 dollars). This value includes all past expenditures ($25 million as of 2017) and predicts future expenditures; accounting for expected changes due to variables like labor and commodities costs and economic inflation. The EAC will continue to be refined as design is finalized, recommendations from the constructability review process are incorporated, risk mitigation is fully implemented, etc.

As a public agency, Denver Water is entirely funded through rates, new tap fees and the sale of hydropower. No tax dollars will be directed towards Denver Water or to the Gross River Expansion Project. The City of Arvada is a partner in the project and will contribute a portion of the funding in exchange for additional water from Denver Water.

If we aren’t in a drought any more, why spend the money to do this?
While Colorado is currently not suffering drought conditions, future droughts are inevitable in this region. Research indicates the strong potential for future water shortages based on drought and community growth. We would not be responsible stewards of this scarce resource if we did not prepare now on this looming supply challenge.
What are the environmental impacts of a project of this magnitude?

The environmental impacts of Gross Reservoir Expansion Project were all identified in the Final EIS and Denver Water has proposed mitigation and enhancements for all of these identified impacts. We have collaborated with others committed to the environmental health of our state to offset the identified environmental impacts, and are proud that in June 2016, the State of Colorado certified that our proposed approach will provide the state with a net environmental benefit to the state’s water quality.

Through the landmark Colorado River Cooperative Agreement, From Forests to Faucets program with the U.S. Forest Service and other collaborative efforts, we are taking unprecedented steps to enhance our watersheds and the communities within them.

A few examples of our commitment to watershed enhancements include:

  • Agreeing with the cities of Boulder and Lafayette to provide an environmental pool in an enlarged Gross Reservoir that will be used to provide enhanced stream flows to a 17–mile stretch of South Boulder Creek below the reservoir.
  • Providing water for current and future West Slope environmental and consumptive use needs.
  • Protecting river flows and enhancing the aquatic environment from the headwaters of the Fraser and Blue rivers at the Continental Divide to the state line.
  • Earmarking $25 million for projects on the West Slope, such as improving rivers and streams and constructing the Berthoud Pass sedimentation pond to improve water quality.
  • Making available 1,000 acre-feet of water each year from Denver Water’s share of the Fraser River for environmental purposes in Grand County, at times and locations requested by Grand County. Denver Water also will release an additional 1,000 acre-feet from Williams Fork Reservoir under specified conditions at the request of Grand County.
  • Partnering with Northern Water and the Colorado Parks and Wildlife to restore a portion of the Colorado River below Windy Gap Reservoir.
  • Providing $16.5 million for the From Forests to Faucets partnership, to be matched by the U.S. Forest Service (total of $33 million), for forest health initiatives in our watersheds.
What wildlife will be impacted by this project and how is that being mitigated?
The Final EIS identified some impacts to wildlife and we have committed to mitigating these impacts. For example, one concern was the impact to the local elk herd on its winter range. To mitigate this impact, Denver Water has purchased land within the South Boulder Creek basin that will be preserved for elk habitat and other environmental needs.
What impact will a higher dam have on aquatic life downstream?

In our one-on-one interactions with individuals in the community and presentations to civic organizations and environmental groups, we often hear the question about whether the water temperatures at the outflow will change and what impact that might have on aquatic life downstream. Here’s how we respond:

  • Environmental studies show that water temperature at the reservoir outflow is generally in the acceptable range for trout today, and will be similar in the future once the expansion is completed.
  • Currently, the biggest threat to aquatic life in South Boulder Creek is lack of water, not water temperatures. As it stands today, there are times of the year when some portions of SBC run at water levels insufficient to maintain aquatic life. An expanded Gross Reservoir will include space for an Environmental Pool, which will be managed by the cities of Boulder and Lafayette. The intent of this is to enable release of water during low flow periods so SBC will have life sustaining flows year-round.
  • While a Multi-Level Outlet Works (MLOW) could increase water temperatures in SBC, the Colorado Department of Public Health and Environment noted on Page 12 of its Rationale for Conditional 401 Certification that “The potential for environmental benefit from the MLOW applies to a relatively short stream reach (about 5 miles in length), and recent data suggest that the water warms noticeably over that distance.”

Ultimately, CDPHE did not include a MLOW as a permit obligation, but did require long-term monitoring of stream temperatures and aquatic life in SBC. CDPHE also recognized all the commitments Denver Water had made to enhance the environment on both sides of the divide and found the Gross Reservoir Expansion would result in a “net environmental benefit” to the state’s water quality.

How many acres of trees will be cut down? What will be done with all the trees that are cut down?
Once construction starts, it should take between six and eight months to remove the 400 acres of trees that will otherwise be inundated by the new high water line. We will make every effort to dispose of the trees responsibly, which includes recycling the timber, selling the wood, allowing people to gather firewood, disposing of wood on-site, and hauling debris to a landfill. We will not use a traditional slash pile-and-burn method because of air quality concerns and regulations, and will make sure we comply with air regulations as we reduce slash piles to ash.
How many property owners might be affected by this project?
Construction impacts from this project vary depending on where the community member lives and their movement habits throughout the area. There are fewer than 100 property owners who will be directly affected by construction – meaning that their homes are directly adjacent to Gross Dam Road. These neighbors will experience truck traffic and potential noise and dust. Those with a view of dam construction activity will see their views change, and have noise and light disruptions. Other members of the community will be impacted indirectly by experiencing traffic disruptions and the frustration of having an ongoing construction project near their homes for a period of about four years. There is one property owner whose land is needed for the project and Denver Water is negotiating a land swap agreement to properly compensate them.
How much higher will the lakefront water level be after the project?
The reservoir will have a high water line that is 124 feet higher than it is today.
If you are raising the reservoir level 124 feet, won’t that submerge most of the private property around it?

All of the property being impacting by the reservoir expansion is within the FERC boundary and owned by Denver Water or the USFS. Twelve acres of private property was acquired by Denver Water in early 2020.

What can be done to lessen the traffic impacts and ensure the safety for residents, especially children, who travel along Hwy 72 every day?
There is a lot that can be done. We plan to work closely with local residents to ensure safety, coordinating with school bus schedules and are evaluating scheduling options that minimize impacts. Safety is our number one priority and we will be responsive to community concerns.
What benefits do the residents of Boulder County, and the nearby property owners of Gross Reservoir receive from this project?

In collaboration with the Cities of Boulder and Lafayette, 5,000 acre-feet of water will be designated for an environmental pool for South Boulder Creek. This pool will provide water during low flow periods and will provide enhanced stream flow to a 17-mile stretch of South Boulder Creek located below the reservoir. Beyond land purchased nearby to preserve elk habitat, riparian areas, wetlands, and other unique features, property owners living near Gross Reservoir receive few direct benefits from this project.

What will the noise impacts be like for the residents that live near the reservoir?
There will be noise impacts to residents surrounding Gross Reservoir during blasting and throughout many phases of construction. Noise from truck traffic and general construction will affect some residents. Project staff will work hard to mitigate noise disturbances as much as possible.
Will there be recreation allowed on the reservoir during construction?

Although we are still designing the project, at this time we anticipate that recreation along the reservoir’s north shore will continue throughout construction, but recreation in other areas, particularly near the dam and in the vicinity of the quarry, will need to be curtailed during construction for safety considerations.

Additional information on recreation during construction available here.


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