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Carter Lake Pipeline

About a third of Boulder’s annual water supply on average comes from Northern Water sources and is treated at the Boulder Reservoir Water Treatment Plant. The Carter Lake Pipeline (formally known as the Southern Water Supply Project II) would change the means of conveyance of water delivered for treatment from an existing seasonally-operated open canal system to a buried, pressurized, steel pipeline capable of year-round operation. The project, which is the result of years of strategic and collaborative planning, would significantly improve the resilience, sustainability and security of the city’s water supply as well as providing enhancements to public health. 

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 participant’s 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 would significantly improve the resilience of Boulder’s water supply by reducing or eliminating the potential for water supply interruptions resulting from flood, wild land fire, system failure and other disasters. For example, the 2013 flood nearly interrupted the city’s drinking water supply by impacting the operation of the city’s two water treatment plants. 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. The Carter Lake Pipeline would replace the canals for source water deliveries to the Boulder Reservoir Water Treatment Plant, which would greatly improve reliability of the Northern Water supply all year round, and would greatly increase the likelihood that at least one of the city’s three water sources would always be available for drinking water supply.


A pipeline would better support the city’s environmental, social and economic goals by reducing energy and chemical consumption and costs associated with delivery and treatment. A pipeline would provide more consistent water to residents, businesses and industries. A pipeline is also in alignment with Boulder Valley Comprehensive Plan guidance that states, “…priority will be placed on pollution prevention over treatment”.  Source water quality at Carter Lake is similar to that of the city’s high quality Boulder Creek water sources, but can degrade to the point of noticeable differences in finished drinking water taste and odor as a result of open canal transport and storage in Boulder Reservoir.  The pipeline’s preservation of source water quality would benefit all customers, including numerous employers who depend on a reliable water supply with predictable quality to produce their products and services.

Public Health and Water Quality

A pipeline would preserve the high quality of the Northern Water supplies by preventing the introduction of natural and man-made pollutants that can enter the source water canals during deliveries to the Boulder Reservoir Water Treatment Plant. Improved source water quality reduces exposure to chemicals, pharmaceuticals and other contaminants that can persist in treated drinking water.  While all Boulder municipal water currently meets applicable water quality standards and is safe to drink, Carter Lake Pipeline would eliminate the effects of canal chemical treatments (for algae) on Boulder’s drinking water and would reduce sediment and associated contaminants accumulated during the water treatment process.  


A pipeline would 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: $46 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 level of service for the city’s water supply is defined by reliability criteria most recently confirmed when City Council adopted the 2009 Source Water Master Plan. The reliability criteria require that Boulder meet droughts of progressively increasing severity with different supply levels as follows:

  • 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

Detailed water supply modeling indicates that Boulder currently needs all of its sources, including Northern Water supplies to meet these criteria. In addition, the 2010 Boulder Valley Comprehensive Plan urban service standards require,” …a sufficient degree of reliability for raw water, treated water, and an efficient transmission/distribution system capacity to meet the demands of the population 24 hours per day.” The Carter Lake Pipeline would improve the city’s ability to meet these policy standards by improving the year-round availability and reliability of Northern Water supplies so that 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. Boulder’s consideration of participation in the Carter Lake Pipeline dates back to an April 2003 Source Water Quality Planning Study. Staff evaluated water treatment and source water protection alternatives for a number of years, and in 2007 the Water Resources Advisory Board supported a measured path forward that included financial participation in Carter Lake Pipeline permitting activities and right-of-way (ROW) acquisition. City Council approved $1,000,000 for the 2008 budget to fund permitting and ROW acquisition activities. In 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. The Water Resources Advisory Board, City Council and a community study group who participated in the 2009 Source Water Master Plan recognized Northern’s importance to Boulder’s water supply and identified the Carter Lake Pipeline as a “no regrets” action subject to funding availability. To accommodate Boulder’s funding strategy, Northern and the other project participants agreed to delay the project in order for Boulder to retire (in 2016) previous utility bonds associated with past projects. Boulder will fund Carter Lake Pipeline construction by issuing utility bonds starting in 2018.  

Environmental Analysis

Economic evaluation of the Carter Lake Pipeline project indicates the project is feasible based on a benefit cost ratio analysis.  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 CH2M 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 CH2M 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.

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