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I I I I I I I I I I I I I I I I I I I DRAFT 8/24/92, Page 27 3. Optimize Use of Flood Control Reservoirs for Water Supply During the 1970's the U.S. Army Corps of Engineers built three major flood control reservoirs in the metro Denver area: Bear Creek, Chatfield and Cherry Creek Reservoirs. Their primary purpose is to capture unpredictable floods that have historically occurred in the late spring and summer months. A secondary purpose of these reservoirs is recreation, and each reservoir maintains a relatively small permanent pool of water for recreational purposes. These reservoirs were originall y designed to protect against a specified level of flood event, typically a 500-year, 30-day flood. In the early 1980's the Corps reassessed its estimates of the amount of storage required to contain such flood events and determined that there was a total of 41,000 AF of excess storage available in Chatfield and Bear Creek Reservoirs that could be reallocated to purposes other than flood protection. Incorporating this amount of storage capacity into an integrated municipal water supply system could significantly increase the yield of that system. This additional storage capacity could be used to capture water storable under new water rights, currently decreed rights, transferred agricultural rights and exchange rights held by municipalities. Previous studies suggest that an additional safe yield of 10,000 to 15,000 AF/year could be generated. a. Dedicate Excess Storage to Water Supply Purposes Reallocated storage in Chatfield and Bear Creek could be operated in several ways. One concept would be to fill and maintain these reservoirs at this additional level of storage and draw them down for water supply only during major droughts; they would serve as reservoirs of last resort. That way their recreational potential would be impacted only infrequently. When needed for water supply, water could be pumped from these reservoirs to nearby treatment plants, or released for downstream municipal diversion. This concept is similar to Aurora's planned use of its Aurora Reservoir. Another concept would be the use of excess storage capacity to capture and store water from the sources described above for use on an annual basis. This mode of operation could result in greater reservoir fluctuation during the summer and fall recreation season. b. Seasonal Use of Flood Control Storage Because the need for these flood facilities occurs primarily during the late spring and through the summer months, there may be an opportunity to utilize flood control facilities for storage of winter flows. An example of this concept is possible in conjunction with the Barr Lake Plan discussed above under municipal first use of agricultural water. Water rights held by the ditch companies involved in the Barr Lake Plan provide for the diversion and storage of large amounts of South Platte River water during the winter months. These flows, or a portion thereof, could be stored in Bear Creek and Chatfield Reservoirs for later release as augmentation water or for direct use or diversion by metro area municipalities. The obvious limiting factor would be the need to schedule releases so that sufficient flood control storage is available by late spring. Issues To Be Addressed 1. The use of storage capacity in Federal flood control reservoirs for non-Federal water supply purposes requires a contract covering both financial and operational aspects, between the United States and a single non-Federal contracting agency. Some state entity such as the Colorado Water Conservation Board would have to act as the clearinghouse agency.