Intent To Prepare a Draft Integrated Feasibility Report/Environmental Impact Statement for the Chatfield Reservoir, CO, Storage Reallocation Project

AGENCY:

Department of the Army, U.S. Army Corps of Engineers, DOD.

ACTION:

Notice.

SUMMARY:

The U.S. Army Corps of Engineers, Omaha District (Corps) is conducting a feasibility study to “reassign a portion of the storage space in the Chatfield Lake project to joint flood control-conservation purposes, including storage for municipal and industrial water supply, agriculture, and recreation and fishery habitat protection and enhancement,” as authorized under Section 808 of the Water Resources Development Act of 1986. The reallocated storage space would be filled using existing water rights. The Colorado Water Conservation Board (CWCB) is requesting the additional storage capacity from the Corps for a consortium of its users in the Denver metropolitan area. The Denver Water Department currently controls all the water rights that account for conservation storage within Chatfield Reservoir. The reservoir serves as the centerpiece for Chatfield State Park. Preliminary studies considered reallocating flood control storage for three storage scenarios reflected by three different raises in the multipurpose pool elevation, currently 5432 feet above mean sea level (m.s.l.): a rise to 5434 feet m.s.l., providing 2,900 acre-feet of storage; to 5437 feet m.s.l., providing 7,700 acre-feet of storage; and to 5444 feet m.s.l., providing 20,600 acre-feet of storage. Operational changes required with a reallocation of flood storage to joint flood control-conservation storage would produce effects on water supplies, downstream flood patterns, recreational opportunities, water quality, and fish and wildlife habitat.

DATES:

Public scoping meetings will be held on:

1. October 26, 2004, 7 p.m. to 10 p.m., Littleton, CO.

2. October 27, 2004, 7 p.m. to 10 p.m., Greeley, CO.

FOR FURTHER INFORMATION CONTACT:

Questions about the study elements should be directed to Mr. Martin D. Timmerwilke, Project Manager, Plan Formulation Section, Planning Branch, U.S. Army Corps of Engineers, 106 South 15th Street, Omaha, NE 68102-1618, phone: (402) 221-4020, email: martin.d.timmerwilke@usace.army.mil.

SUPPLEMENTARY INFORMATION:

1. Background. The Corps operates the Chatfield Reservoir located near Denver, Colorado to provide flood protection for the greater metropolitan area. The reservoir is located on the main stem of the South Platte River; Plum Creek also contributes flow to the reservoir. Congress authorized construction of the reservoir under the Flood Control Act of 1950. The Corps began construction in 1967, and dam closure occurred in 1973. The authorized uses for Chatfield Reservoir are flood control, recreation, water supply storage, and fish and wildlife enhancement.

Under the Corps' current operating plan, conservation storage is filled by Denver Water Department water rights and used for municipal and industrial uses. The State of Colorado, Department of Natural Resources, Division of Parks and Outdoor Recreation has a park and recreation lease from the Corps for 5,381 land and water acres, including the area covered by Chatfield Reservoir. Recreation facilities at Chatfield State Park include hiking and biking trails, campgrounds, picnic areas, a stable, boat ramps, a beach, and a marina. Chatfield State Park receives over 1.5 million visitors annually and provides habitat for numerous wildlife species. The Corps has also leased portions of the Chatfield Project property to the Denver Botanical Gardens for public recreation and to the Colorado Division of Wildlife for fish production and rearing areas. Three irrigation ditches located at the base of the dam supply water to users in Aurora, Englewood and Highlands Ranch.

Population growth within the Denver, Colorado metropolitan area continues to create a demand on water suppliers. The CWCB, representing a number of smaller municipal water user groups, requested that the Corps consider reallocating space to accommodate additional conservation use. Reallocating storage capacity within the reservoir requires the preparation of a Reallocation Feasibility Report. The Feasibility Report will be completed in conjunction with an integrated environmental impact statement (EIS) developed for the project.

Chatfield Reservoir has a total gross storage of 350,043 acre-feet. This storage is distributed into four zones defined by elevation. The inactive zone extends from the bottom of the reservoir, elevation 5377 feet m.s.l. to 5385 feet m.s.l., with a storage volume of 28 acre-feet. The multipurpose zone extends from 5385 feet m.s.l. to 5432 feet m.s.l., with a storage volume of 27,018 acre-feet. The flood control zone extends from 5432 feet m.s.l. to 5500 feet m.s.l., with a storage volume of 206,729 acre-feet. The surcharge zone extends between 5500 feet m.s.l. to 5521.6 feet m.s.l., with a storage volume of 116,268 acre-feet.

Chatfield Reservoir is managed to maintain the level within the multipurpose pool from Memorial Day through Labor Day. Denver Water Department holds all of the rights for the water up to the top of multipurpose pool, 5432 feet m.s.l. The State Engineer's Office submits requests to the Corps for releases from the reservoir on behalf of Denver Water Department. Once the pool rises above 5432 feet m.s.l., the Corps is responsible for the management of water in the flood control pool. The Corps works to reduce the flood control pool as quickly as possible within the constraints established in Chatfield Reservoir's Operating Plan. Releases from Chatfield Reservoir are coordinated with releases from Cherry Creek and Bear Creek reservoirs. The Corps attempts to limit the releases so the flow of the South Platte River at the Denver gauge remains less than 5,000 cubic feet per second.

Operational changes would be required with a reallocation of flood control storage to joint flood control-conservation storage and would produce effects on water supplies, downstream flood patterns, recreational opportunities, water quality, and fish and wildlife habitat. In determining whether to reallocate storage within the reservoir and change operational regimes, the Corps must comply with requirements including but not limited to the Endangered Species Act, the National Environmental Policy Act, the National Historic Preservation Act, and the Clean Water Act.

2. Proposed Action. The Corps is studying the feasibility of reallocating some flood control storage capacity in Chatfield Reservoir to joint flood control-conservation purposes, which include water supply. The reallocation is needed to enable the CWCB to provide water to local users for municipal, industrial, agricultural, recreational, and fishery uses in response to population growth in the greater Denver metropolitan area.

3. Alternatives Considered. The Corps, working with the CWCB, has identified and conducted reservoir routing studies on three alternative increases in the multipurpose pool elevation for further consideration: A raise to 5434 feet m.s.l., providing 2,900 acre-feet of storage; to 5437 feet m.s.l., providing 7,700 acre-feet of storage; and to 5444 feet m.s.l., providing 20,600 acre-feet of storage. The three elevations considered in the preliminary study would be anticipated to have different levels of impacts on recreational facilities as well as on fish, wildlife and vegetation resources. The Corps' no action alternative will also be considered.

The three pool-raise alternatives initially identified would require changes to the operation of the reservoir, and would have different effects on the existing recreational facilities and use levels within Chatfield State Park. If the multipurpose pool is raised to 5434 feet m.s.l., recreation impacts could be mitigated without relocating the existing structures. Raising the multipurpose pool to 5437 feet m.s.l. would require expenditures to keep the existing recreational features operational. Raising the multipurpose pool to 5444 feet m.s.l. would require relocating most of the existing recreational facilities and infrastructure to other, mostly nearby, sites in Chatfield State Park. These alternatives may also differ in the need for, and type of, modifications to existing project structures. The Corps has not yet defined specific operational regimes for the pool-raise alternatives. Additional alternatives, which could include different storage volumes and varying operational regimes, could be developed during the scoping and evaluation process.

The demand for water within a reallocated storage pool would depend on the holders of the water rights used to fill the storage space. Potential users fall into one of four groups: Municipal water suppliers, entities requiring augmentation water, entities concerned with maintaining minimum instream flows in the South Platte River, and water users for municipal, industrial, and conjunctive uses. How the water within the reallocated storage pool would be withdrawn would depend on the objective of the water users. A preliminary study of user patterns evaluated five demand scenarios that corresponded to different target release schedules as follows:

a. Supplying municipal water, with release schedules based on historic data provided by Denver Water Department.

b. Augmenting out-of-priority depletions, primarily for irrigation.

c. Minimum in-stream flows throughout the year within the South Platte River.

d. Municipal, industrial, and conjunctive use of storage within Chatfield Reservoir combined with a groundwater source.

e. Mixed use, where the reallocated storage could be used for a combination of the above uses.

4. Scoping/Public Involvement. The scoping process will provide information about the reallocation study to the public and serve as a mechanism to solicit agency and public input on alternatives and issues of concern. Two public scoping meetings are currently planned. The specific locations of the meetings will be provided in news releases issued at least 2 weeks prior to the meetings. These meetings will be conducted in an informal setting designed to present information about the reallocation study and to answer questions and accept comments from the public. The Corps invites other Federal agencies, Native American Tribes, State and local agencies and officials, private organizations, and interested individuals to attend one of the scoping meetings and provide comments. Scoping comments will also be accepted by mail, phone, or e-mail during the preparation of the Draft Feasibility Report/Draft EIS. The Draft Feasibility Report/Draft EIS will be circulated for public review and comments. It is estimated that a Draft Feasibility Report/Draft EIS will be completed in 2006.