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Carter Lake Pipeline

On average, about a third of Boulder’s annual water supply comes from Northern Water sources and is treated at the Boulder Reservoir Water Treatment Plant. 

The Carter Lake Pipeline (also known as the Southern Water Supply Project II) will change how water is delivered to the treatment plant from an existing seasonally-operated open canal system to a buried pipeline capable of year-round operation. 

Construction of the pipeline began in fall 2018 and is scheduled to be completed in early 2020. The Northern Colorado Water Conservancy District (Northern Water) is managing the project on behalf of the participants: City of Boulder, Berthoud, Left Hand Water District and Longs Peak Water District. 

The project will significantly improve the resilience, sustainability and security of the city’s water supply as well as provide enhancements to public health. 

Construction Impacts near Tom Watson Park and Coot Lake

Although the majority of construction for this project has taken place outside the City of Boulder, a portion of the pipeline is aligned along 63rd Street north of Highway 119 (the Diagonal) and through Tom Watson Park.

Pipeline construction in nearing completion, with full project completion in March 2020. Updates will be posted to this web page, and more information can be found on the Northern Water web page.

Construction notices are posted in impacted areas to inform users about upcoming activity. 

Project Description

The Carter Lake Pipeline will be a 20-mile-long, roughly 3-foot-diameter, buried steel pipeline and will deliver water from Carter Lake Reservoir to participants' treatment facilities ending at the City of Boulder’s Boulder Reservoir Water Treatment Plant. The pipeline will be owned and operated by Northern Water and will deliver Colorado-Big Thompson (CBT) and Windy Gap water to Longs Peak Water District, Left Hand Water District, and the City of Boulder. The participants will each own a share of capacity in the pipeline and will provide all necessary funding for construction and operation and maintenance. Pipeline capacity information in cubic feet per second (cfs) is as follows:

  • City of Boulder: 32 cfs
  • Left Hand Water District: 11 cfs
  • Longs Peak Water District: 3 cfs

Total: 46 cfs 

Project Benefits


The Carter Lake Pipeline will reduce the potential for water supply interruptions during floods, wildfires, system failures and other disasters. For example, the 2013 flood nearly interrupted the city’s drinking water supply. The Boulder Reservoir Water Treatment Plant was inoperable due to damage to the canals that deliver Northern Water to the plant, while the Betasso Water Treatment Plant was nearly shut down since it was almost inaccessible for chemical and fuel deliveries due to damage to Highway 119 in Boulder Canyon.

With a buried pipeline instead of an open channel carrying water to the Boulder Reservoir plant, the city will have a reliable Northern Water supply all year round. The pipeline will also increase the likelihood that at least one of the city’s three water sources will always be available for drinking water supply, even during emergencies or disasters.


By preventing contaminants from entering the water, the Carter Lake Pipeline will reduce energy and chemical consumption and costs associated with drinking water delivery and treatment. The pipeline aligns with Boulder Valley Comprehensive Plan guidance that states, “…priority will be placed on pollution prevention over treatment."   

Public Health and Water Quality

The Carter Lake Pipeline will help preserve the high quality of the water from Northern Water supplies as it travels to the Boulder Reservoir Treatment Plant. While all drinking water must adhere to water quality standards, there can be noticeable differences in drinking water taste and odor as a result of open canal transport. 

The pipeline will prevent natural and man-made pollutants from entering the water, thereby reducing exposure to contaminants that can persist in treated drinking water. The pipeline will also eliminate the need for canal chemical treatments for algae and reduce sediment and associated contaminants accumulated during the water treatment process.  

The pipeline’s preservation of source water quality will benefit all customers, including numerous employers who depend on a reliable water supply with predictable quality to produce their products and services.


The pipeline will protect the water supply from intentional or accidental contamination risks. ​

Project Cost, Funding and Timeline

The following table summarizes project costs and Boulder’s funding plans.

Project Cost Permitting,Right-of-way Acquisition,Design and Construction  
  • Total Project Cost: $44 million

  • City of Boulder Cost: $35 million (32 cfs capacity)

  • Left Hand Water District Cost: $10 million (11 cfs capacity)

  • Longs Peak Water District Cost: $1 million (3 cfs capacity)

Cost allocation is primarily appropriated among participants based on percentage of overall capacity owned. However, each participant will completely fund project features for which it is the only user.

Annual Operations and Maintenance Costs (Estimated at 0.5 percent of Construction Costs
  • Total Annual Operations and
    Maintenance (O&M) 
    Cost: $211,000

  • City of Boulder Cost: $158,000

  • Left Hand Water District Cost: $47,000

  • Longs Peak Water District Cost: $6,000

In any given year, any unused cash balance for the pipeline O&M assessment carries over and is subtracted from the subsequent year’s O&M assessment. 

Funding Strategy and Timing

Carter Lake Pipeline is scheduled for construction in 2018 but was originally scheduled to be completed several years earlier. Northern and the other project participants agreed to delay the project in order for Boulder to retire previous utility bonds associated with past projects. Boulder will fund Carter Lake Pipeline construction by issuing utility bonds starting in 2018. 


Water Supply Policy and Strategic Planning

Water Supply Policy

The 2009 Source Water Master Plan requires the City of Boulder to meet all water supply needs, except during periods of severe drought:

  • Up to 20-year drought - meet all supply needs
  • Up to 100-year drought - reduced supply (mostly outdoor watering restrictions)
  • Up to 1,000-year drought - meet essential (indoor and emergency) supply needs

Boulder currently needs all of its water sources combined, including Northern Water supplies, to meet these criteria. The Carter Lake Pipeline will improve the city’s ability to meet these policy standards by improving the year-round availability and reliability of Northern Water supplies. The pipeline will also help ensure Boulder’s drinking water needs are met even if the city’s other two water supply sources were unavailable due to a natural disaster or unplanned system outage.

Strategic Planning

Carter Lake Pipeline is the result of years of strategic and collaborative planning. The following timeline highlights some of the key milestones:

  • April 2003: Source Water Quality Planning Study completed. Staff continue to study various alternatives for source water protection and treatment.
  • 2007: Water Resources Advisory Board supports financial participation in Carter Lake Pipeline permitting activities.
  • 2008: City Council approved $1 million to fund permitting and right-of-way acquisition.
  • 2009: City Council approved the Source Water Master Plan, which included the Carter Lake Pipeline as part of the Water Utility Fund 20-year CIP.
  • May 2018: City Council authorized issuance of Water and Sewer Revenue Bonds, the majority of which will pay for the Carter Lake Pipeline. Favorable bids resulted in an almost $1 million savings over the anticipated cost of the project.

Environmental and Economic Analysis

To help decide whether to move forward with the pipeline project, the city and consultants conducted a cost-benefit analysis. The analysis focused in part on the risk of losing the ability to treat water at both treatment plants. If Boulder ever lost the ability to treat water at both treatment plants, as almost occurred during the 2013 flood, the impacts to the community would be significant. Some of the social, political and economic consequences would be severe and in some cases irreversible. Essential services such as hospitals and fire protection and businesses such as restaurants and many large commercial operations cannot function without water. According to a 2016 estimate, the economic impact of a one-month outage could be as much as $186,000,000. Dividing this outage cost by the life cycle cost of the pipeline results in a benefit-cost ratio of 4.

The cost-benefit estimate is based on generalized risk analysis methodology and results in the economic impact of lost water supply at $6.2 million per day. Using economic data more specific to Boulder indicates the economic impact of lost water supply could be as much as $10 million per day. It should be noted that loss of water supply for even one day is unacceptable according to Boulder Valley Comprehensive Plan policies.