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i Executive Summary climatological variation. Annual precipitation ranges from more than 40 inches in the high alpine zones in the east and south to negligible amounts in the more and western portion of the ' basin. There are 23 currently operating streamflow gages in the Yampa River basin, 15 of which are in Colorado above the confluence of the Yampa and Little Snake Rivers. Another 60 gages have operated over various periods in the past but have been discontinued. Of all these gages, only the Steamboat Springs and Maybell gages on the Yampa River mainstem encompass the 1930 through 1982 hydrologic period used in this study. Figure S-1 ' schematically depicts the distribution of flows and depletions in the Yampa River basin above the Little Snake River confluence. ' Water Rights The waters of the Colorado River basin have been apportioned for use between the upper and lower basin states by the Colorado River Compact of 1922. The upper basin allocation has been further apportioned among the states of Arizona, Colorado, New Mexico, Utah, and Wyoming by the Upper Colorado River Basin Compact of 1948. Article XIII of the Upper ' Basin Compact specifically addresses the apportionment of water of the Yampa River between Colorado and Utah, stipulating that Colorado will not cause the Yampa River at the Maybell gaging station to be depleted below an aggregate flow of 5 maf for any period of 10 consecutive years. The average annual historical flow at the Maybell gage is 1. 135 maf and ' the minimum historical 10 year sum is 9.1 maf. Hence, Colorado has not even closely approached its depletion entitlement under the Compact. ' Although the Yampa River basin is relatively undeveloped in terms of potential water use, it is considered over-appropriated from a Colorado water rights perspective. The sum of decreed (absolute and conditional) direct flow and storage water rights is greater than the i average annual flow of the river as measured at the Maybell gage. Many of these rights are conditional and have not yet been exercised; others have not routinely been exercised to their full decreed amounts due to the relatively low demand for water in the basin. As a result, the mainstem water rights of the Yampa River have not historically required administration by the Colorado State Engineer. It is commonly believed that the controlling water rights on the river are the conditional water rights for the Juniper Project. Any discussion of relative water rights priorities in the basin generally comes down to whether the rights in question are senior or junior to the Juniper rights. In recognition of this fact, River District has obtained decree amendments and agreements with water users in the basin subordinating the priority of the Juniper rights to their junior water rights and uses. One of these agreements (the "general subordination") stipulates that the Juniper rights will not place a call for water rights administration until upstream consumption under junior water rights exceeds 65,000 af. The other subordination agreements address specific junior water rights and uses. One of the principal purposes of the current study is to assess the implications, ' particularly with regard to future basin water needs, of transferring a portion of the Juniper Project water rights to instream flow use to assist in the recovery of the endangered fishes. There are two broad institutional strategies by which this transfer might occur, each of which i raises its own set of technical and institutional issues. One strategy would be to transfer the entire amount of the Juniper water right to an instream flow right with protection of existing junior uses and future basin water development needs occurring solely through subordination of the instream flow right to those uses and I S-3