Lake Dillon and Denver Water Operations of Blue River Decree
AWRA Colorado Summer Field Trip – July 17, 2009
The AWRA Colorado Section’s annual summer field trip for 2009 was a tour of Lake Dillon on July 17th, and included discussions about the history of the reservoir, water operations and recreational issues surrounding the reservoir, and potential cooperative water supply projects being considered to better utilize Lake Dillon water supply. Tour guides were Dave Bennett (Denver Water Planning Department), Dave Fernandez (Lake Dillon caretaker), and Scott Hummer (Water Commissioner for the Blue River Basin, District #36). A beautiful July day was spent cruising around the lake on pontoon boats while discussing water issues for Lake Dillon, the Blue River Basin, and beyond to the Colorado River Basin.
The day began with a discussion of Dillon Marina’s inspection process for invasive species (coaga and zebra mussels). Bob Evans, the Dillon Marina manager, explained that Lake Dillon is the largest uninfected water body in Colorado as a result of a strict inspection and decontamination protocol. The marina uses a self-contained decontamination machine, which uses 190 degree Fahrenheit water at a rate of up to 5 gpm to clean boats prior to launching at the marina. A giant mat is placed under the decontaminated boat to capture decontamination water, which is beneficially used for purposes such as dust control at construction sites near Dillon. The decontamination machine cost the Town of Dillon approximately $30,000.
Dillon Marina’s Boat Decontamination Machine
Leaving the marina on pontoon boats, Dave Bennett, discussed the history of Lake Dillon. Denver Water considered water supply options in the 1920s as the S. Platte River became over appropriated. The Moffat Collection System was built in the 1930s, and Denver Water constructed the Roberts Tunnel from 1946 through 1962. Lake Dillon was then completed in 1963 with a capacity of 254 kAF. The reservoir was built over the historic Town of Dillon (population 1,200), and the town was moved to its current location on the east side of the reservoir. At its lowest storage levels (e.g., 2002), the historic road of the Town of Dillon can still be seen. Lake Dillon’s contributing drainage area includes the Blue, Snake, and 10 Mile rivers and includes a total of 335 square miles. About 25 percent of Denver Water’s total water supply comes from the reservoir, with approximately 60 kAF a year being diverted through the Roberts Tunnel. The reservoir is solely used for water supply storage, with no extra capacity available for flood control. Releases of up to 4,000 cfs can be made through the outlet works, and spills through the spillway can be up to 10,000 cfs. The highest outflow from the reservoir (2,010 cfs) was made in 1984, but the maximum release desired to prevent flooding of downstream properties is 1,800 cfs.
Dave Bennett, Denver Water Planning Department
Lake Dillon operations consider the water needs of Denver Water, recreational uses on the lake, and rafting and fishing downstream on the Blue River. Denver Water fills Lake Dillon in Spring using its 1946 Blue River decree. They typically use their S. Platte River decrees and supplies early in the Spring, followed by Dillon water later in the season. Using their S. Platte supplies first is not required under their Blue River decree, but it allows Denver Water to minimize their West Slope demands. Mediation is currently underway to protect West Slope recreational interests, but also preserve Denver’s supply from their Blue River decree. The 1946 Blue River decree is junior to the 1922 Colorado River Compact, and as a result, Lake Dillon could be called out in the case of a Compact call. Denver Water is looking to buy agricultural rights on the West Slope that would be senior to the Compact. They would use these senior rights to facilitate a water bank to be used in the case of a Compact call. Other issues with the Blue River decree that were discussed included the often debated definition of what is included in the “Denver Metro area.” Denver Water’s Blue River decree permits water use for municipal demands within the Denver Metro area, but an exact specification of what is included in the metro area has never been established.
Recreational uses of Blue River water include boating and fishing on Lake Dillon, and rafting, kayaking, and fishing downstream of the dam. Denver Water attempts to fill Lake Dillon for both the Dillon and Frisco marinas with the goal of getting the marinas into business by Memorial Day. The marinas struggle in dry years, for example, the Frisco marina had operational difficulties in the drought of 2002. Rafting on the Blue River typically runs from June through about July 4th, with a minimum of about 500 cfs needed for decent rafting conditions. Rainbow trout fishing downstream of the Dillon Dam is another recreational consideration for Denver Water.
Cruising Lake Dillon on Pontoon Boats
A conceptual conjunctive use project was discussed that would involve cooperative use of Blue River supply for Denver, Aurora, and South Metro entities. Denver Water projects about 10 kAF per year of their Blue River supply that would not be used at buildout and would otherwise flow down the S. Platte River. This excess could be diverted using existing infrastructure (e.g., the Cherry Creek Pipeline and/or Aurora’s Prairie Waters Project), and would not require new construction or environmental permits to implement. This project is currently being termed the “Wise Project” and will probably continue to be considered as promising for future wise use of Front Range water supplies.