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.. . ~...., ~,~ . . The Colorado Water Conservation Board and. the Bureau of Reclamation have jointly made a preliminary evaluation of the eco~ nomic feasibility of restoring ~he channels of the Rio Grande and its,tributaries in Colorado 0 This evaluation consisted of deriving , preliminary estimates of construction costs for restoring the stream channels; estimating-the annual operation, maintenance, and. replace- ment costs; estimating the potential savings of water and the basin- wide benefits resulting therefrom; deriving preliminary benefit-cost ratios; and. determining the expenditures required for more detailed studies of feasibility grade investigations 0 No detailed field investigations have been made. The pre- liminary evaluation of construction costs is based upon an aerial reconnaissance, the judgment of engineers of the Colorado Water Con- servation Bpard and the Bureau of Reclamation who are familiar w~th the area, and upon readi4' available 'hydrologic and. topographic data. In restoring the channels of the main'river and "tributaries, the present riverbed elevations would be retained to conform with the 'elevation requirements of the present irrigation systems. The river channels would not be deepened because they could then act as major drains, thus disturbing the hydrologic balance of the practice of subirrigation in the Valley 0 The channel restoration works would not be designed for flood protection purposes because major flood control in the San Luis Valley can be economically achieved through operation of the existing and. potential reservoirso The annual operation, maintenance, and replacement costs have been estimated from the costs experienced in maintaining the Rio Grande channel in the Middle Rio Grande Valley, with appropriate adjustments for differences in sedimentation and. vegetative conditions in New Mexico and Coloradoo The reduction in water losses that would result from channel restoration was estimated from the results of past investigations of rates of evapotranspiration in the, San Luis Valley and extensive studies of reducing water losses in the Rio Grande Basin above Fort Quitmano The value of water savings to the basin was kept conservative4' 11.ow at $60 per acre-foot 0 This is an average value of water for irrigation in the Upper Rio Grande Basin derived in other Bureau of Reclamation economic studies. The San Luis Valley is an area of major importance for recreation, fishing, production of ducks, and the maintenance of both large and small bird and mammal lifeo Any required mitigation mea- sures for fish and wildlife purposes, as well as benefits from enhancement of fish and wildlife and recreation purposes, were recognized but not studied in this preliminary evaluation. In future, more detailed studies, the purposes of channel restoration, prevention of water loss, fish and wildlife mitigation and enhance- ment, and recreation enhancement would be combined to attain optimum development for all purposeso 2