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Reservoir "Rules"

Westerners prudently store water from each spring's abundant runoff to use throughout the year. Colorado now has about 2,000 reservoirs statewide, which the Division of Water Resources must administer. In an attempt to informally codify the state's reservoir administration practices, Water Division 1 Assistant Division Engineer Claudia Engelmann and Division Engineer Jim Hall, along with Water Division 5 Division Engineer Alan Martellaro, assembled a set of guidelines intended to provide a common starting point for the many difficult decisions DWR staff must make every day. The guidelines are currently being reviewed by State Engineer Dick Wolfe. Here is a sampling of the issues covered:

One-fill rule—Established by historical court opinions, the ‘unwritten’ one-fill rule limits a storage water right to filling a reservoir once in any given seasonal year. A seasonal year typically begins Nov. 1, but for many municipal reservoirs, April 1 is the start date.

Second fill—A decreed refill right allows the owner of the water storage right to begin to fill a second time once available space is made in the reservoir. This is only allowed when the refill right, which often has a later, more junior date, is in priority.

Paper fill—Using this accounting method, division engineers document when a storage right is fulfilled on paper, even if it has not been physically filled. Some reservoirs have more than one owner or more than one decreed storage right with different priority dates. They are required to take the most senior water first. However, they may be allowed to fill first under their junior water right while division engineers use a paper fill to book the water against the senior right. That way, they keep track of how much less the owner can later call under the senior water right, if the junior right comes out of priority. Engelmann acknowledges that ‘the accounting gets extremely complicated when you have to track the different priorities and owners of water.’
Why the complication? According to Martellaro, who has served as Division 5 engineer for almost 9 years, a common example would be if a senior water right is limited under its decree to irrigation uses but the junior water right's decree is more flexible. In this case, the owner may consider the junior right more valuable and elect to store that water first. Martellaro says this is becoming more common as ‘people are coming up with good, creative ways to better use what water's out there.’

‘Owe-the-river’ account—Some reservoirs are literally built on the stream. When an on-stream reservoir's storage right is out of priority, DWR staff attempt to administer the reservoir as if it didn't exist, says Engelmann. To mimic the natural streamflow and maintain peaks in flow through the system, they track inconsistencies through an ‘owe-the-river’ account. If the reservoir releases too little water one day, it must release more the following day to compensate.

Exchanges and substitutions—Exchanges and substitutions may be made between reservoirs or between a reservoir and a direct-flow diversion. ‘A substitution is when we make a release from one reservoir for the purposes of another,’ says Martellaro. ‘It's not done at the same time, whereas an exchange happens at the same time.’
A frequently occurring substitution in his division occurs between Green Mountain Reservoir and Wolford Mountain or Williams Fork Reservoir when Denver Water's Dillon Reservoir fills out of priority. By state statute, an upstream reservoir can fill out of priority if allowed by the State Engineer, but if a downstream reservoir with a more senior right doesn't end up filling, the upstream reservoir must pay the water back. If Green Mountain Reservoir, downstream from the more junior Dillon Reservoir, doesn't fill, Denver often pays back the water directly to the mainstem of the Colorado River where Green Mountain's water would otherwise be destined. In-lieu of Green Mountain sending water there to meet a call on the river, Denver would use its Williams Fork Reservoir or its interest in the Colorado River Water Conservation District's Wolford Mountain Reservoir to make releases to pay back Green Mountain indirectly. Division 1 has adopted its own requirements that must be met before any reservoir on the South Platte's mainstem can store out of priority.

An exchange might occur to allow a more junior water right to continue diverting even when out of priority by replacing the same amount of water at the same time from another source. Another Division 5 example is when Denver releases water from Williams Fork Reservoir into the Colorado River so that it can continue out of priority diversions through Roberts Tunnel to the city.

Again, Martellaro says, ‘Substitutions are becoming more and more common to make better use of what we have."

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