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2-10 I I I I I I I I I I I I I I I I , I I Another case, decided later that year, went even further. In Board of County Comm'rs. of the County of Arapahoe v. Upper Gunnison River Water Conservancy Dist.,37 the court approved a water right for reservoir storage which is used both for irrigation and to enhance a downstream fishery. This case is complicated, even in the simplified version presented here, but it is worth the effort to understand because of the importance of the precedent it sets. The Taylor Park Reservoir was built on the Taylor River, a tributary of the Gunnison River, in the 19308 to firm up water supplies on the Gunnison Tunnel, some 100 miles downstream, which had been constructed by the Bureau of Reclamation at the turn of the century.38 Resulting flow fluctuations below Taylor Reservoir proved harmful to the trout fishery on the Taylor River. Following completion of the Aspinall Unif9 downstream on the Gunnison in the 1970s, local residents and businesses suggested that water for the Gunnison Tunnel might be provided more efficiently out of the Aspinall Unit (which had considerable uncontracted storage space and was positioned not far upstream of the Gunnison Tunnel). This, they figured, would allow the Taylor Park Reservoir to be operated in a manner less harmful to the fishery. In 1975 the parties with an interest in the reservoirs entered into an exchange agreement to implement this idea. The reservoirs were then so operated to the great benefit of the Taylor River fishery and the area's growing recreational economy. No one, however, sought to confirm the arrangement with the Colorado Water Court. That did not matter until 1986, when two separate development schemes were proposed to divert water from the Gunnison Basin to Colorado's Front Range communities. At that point the Upper Gunnison River Water Conservancy District, a signatory to the 1975 contract, filed claims for water rights consistent with how the project then was being operated.40 Specifically, the Upper Gunnison District sought a water right for a "second filling" of the Taylor Park reservoir.41 Under the law in most western states, a reservoir storage right may be exercised through only one physical filling per year. Thus, once a reservoir is filled and drawn down, it may not fill again during the same year, to. the extent other users (including juniors) call for the water. If a second filling is desired, the appropriator must obtain an additional water right for the second filling. Here is the tricky part. The Taylor Park Reservoir did not physically fill more than once a year. But enough water was passed through the reservoir that it could have filled twice, if it had been operated that way. I~ order to enhance the fishery below, however, the reservoir was operated so as to bypass much of the flow. Consequently, it filled more slowly that it otherwise would have. In order to make out a claim for a second filling, the Upper Gunnison District argued that all water which it was in priority to store should be counted toward the firstsfilling. Thus the first filling was accomplished when the reservoir was, say, half full.