## Facts

- Colorado consolidated two statutory water adjudication proceedings in Water District No. 36 (Colorado River basin) involving irrigation rights and non-irrigation rights.
- Denver pursued a long-term municipal supply program that included transmountain diversions from Western Slope tributaries of the Colorado River to the South Platte system.
- Denver investigated and planned potential diversions from the Blue River beginning in the 1920s, but did not begin actual construction of its Blue River tunnel/diversion works until 1946.
- The Northern Colorado Water Conservancy District sought rights for downstream storage and related works associated with what became Green Mountain Reservoir, asserting a 1937 priority date based on governmental and district actions tied to regional project development.
- The trial court decreed priorities giving the Northern District a 1937 priority for its storage project on the Blue River system and assigning Denver a junior 1946 priority for its Blue River transmountain diversion.
- Denver and aligned Eastern Slope entities sought appellate review, arguing for an earlier priority date based on relation-back to early planning and on a “unified system” theory tied to Denver’s earlier adjudicated rights on other Western Slope streams.

## Issues

1. Whether Denver’s Blue River appropriation could relate back to early surveys, planning, and investigations from the 1920s to obtain a priority date senior to 1937.
2. Whether Denver’s previously adjudicated appropriations on different streams (Fraser River and Williams Fork) could extend an earlier priority date to Denver’s Blue River project under a unified municipal system theory.
3. What combination of intent, overt acts, and diligence is required to initiate and maintain an appropriation date for a large, multi-year water project.
4. Whether Denver’s Blue River diversion right was senior or junior to the Northern District’s 1937 storage right.

## Decision

- The Colorado Supreme Court affirmed the district court’s priority determinations as between Denver and the Northern District.
- The court rejected Denver’s attempt to backdate its Blue River priority to the 1920s based on planning and investigations.
- The court held Denver’s Blue River right did not become legally effective until substantially later, supporting the junior priority date tied to the commencement of construction in 1946.
- The court upheld the Northern District’s 1937 priority for its storage project as senior to Denver’s Blue River diversion.

## Legal Principles

- An appropriation is initiated by intent to appropriate coupled with an overt physical act directed to the specific diversion, followed by reasonable diligence toward completion and beneficial use.
- Long-range planning, engineering studies, and exploratory investigations, without stream-specific physical acts toward diversion works, do not establish an appropriation date.
- Appropriation rights are generally source- and stream-specific; an adjudicated priority on one stream does not automatically confer a priority on a different stream, even if both are ultimately tributary to the same river system.
- A unified municipal system concept does not permit carrying an earlier priority from one stream to another absent legally sufficient acts establishing an appropriation on the second stream.

## Conclusion

The court upheld a strict application of prior appropriation requirements by refusing to treat Denver’s decades of planning as the initiation of a Blue River appropriation, holding Denver’s Blue River priority junior to the Northern District’s 1937 storage right because Denver did not undertake stream-specific, diligent construction acts until 1946.


City & County of Denver v. N. Colo. Water Conservancy Dist., 130 Colo. 375, 276 P.2d 992 (Colo. 1954)