Chatfield Reservoir Environmental Impact Study

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by Tom Browning

Background: The Chatfield Reservoir Storage Reallocation Study (Study) is currently underway to assess the feasibility of reallocating (e.g. converting) some of the existing storage space in Chatfield Reservoir, which is located along the South Platte River south of Denver. The study includes a Feasibility Report and an Environmental Impact Statement (FR/EIS) and is being conducted by the U.S. Army Corps of Engineers (Corps), which owns and operates Chatfield Reservoir and Dam. The main purpose of the reservoir is for flood control, but it also provides storage space for municipal, industrial, agricultural, environmental, and recreational uses. The property around Chatfield Reservoir is leased from the Corps and managed by Colorado State Parks as a significant recreational area. The Colorado Water Conservation Board (CWCB) is the non-federal sponsor of the Study and has entered into a Feasibility Cost Share Agreement with the Corps to complete all Study tasks. In all, the total Study budget is projected to be on the order of $4.5 million. The federal-state partnership is working closely with a group of 15 water providers along the Front Range, in addition to a robust group of Special Technical Advisors. The overall collaboration, cooperative spirit, and innovation by the entire Study coalition are exemplary and may provide a model for future similar projects in Colorado. Additional storage in the joint flood control/multi-purpose pool at Chatfield Reservoir could provide a much needed water supply scenario by capturing flows, including certain renewable (return) flows, from the South Platte River and Plum Creek, and help reduce dependence on nonrenewable groundwater. If approved, a reallocation would help regional water providers meet demand for municipal and industrial needs in response to population growth in the region, provide additional water supplies for agricultural and recreational uses, and enhance fisheries and wildlife habitat. The purpose of reallocation is to convert pool space from one type of storage use to another. In this case, a reallocation would change water storage space in Chatfield Reservoir that is currently reserved for flood control purposes to storage space for joint flood control and multi-purpose needs. This is achieved by increasing the existing multipurpose pool thereby allowing for additional multipurpose water supply. The reallocation could occur without the need for any physical changes to the dam, outlet works, or emergency spillway. Essentially, changes to the reservoir operating criteria in combination with a revised hydrology study (Antecedent Flood Study that has already been completed and approved by the Corps) serve as the foundation blocks for the ability to consider reallocated storage space. Interested water providers have investigated Chatfield Reservoir as a potential means of storing surface water and helping to meet demands for municipal and industrial water supplies in response to population growth in the region and in response to the need for reducing dependency on non-renewable groundwater. In addition, the project would increase water supplies for agricultural and recreational uses, and would enhance fisheries and wildlife habitat.

Current Reservoir Use: Chatfield Reservoir and Dam were built in 1975 by the Corps at the confluence of the South Platte River and Plum Creek, to provide flood protection for the greater Denver metropolitan area.

The reservoir currently has the ability to store just over 350,000 acre-feet (AF) of water without overtopping, with a majority of that storage space reserved for flood control. Currently, Denver Water is the only provider with rights to store water in Chatfield’s joint flood control/multi-purpose pool. Denver Water’s use of the pool is subject to certain conditions for maintaining water levels for recreation and conservation. Releases from Chatfield Reservoir are coordinated by the State Engineer’s Office, based on Colorado water law and the demand for water supply, while minimizing water level fluctuations during the recreation season (Memorial Day to Labor Day). Once the reservoir level rises above 5,432 feet m.s.l., the Corps is responsible for the release of water from the flood control pool.

Regional Water Demand: Population growth along the Front Range continues to create a demand for new water supplies. Colorado’s population is projected to increase by 65% between 2000 and 2030 according to the CWCB’s Statewide Water Supply Initiative (SWSI). The SWSI report estimates that by 2030, water demand in the South Platte River Basin will exceed supply by 22%, leaving 90,700 acre-feet (AF) of unmet needs. The unmet needs (a.k.a. the “gap”) estimate actually assumes that a number of vital water supply initiatives and projects will be implemented between now and 2030. There is also a strong need for additional water supplies for the agricultural community in the South Platte Basin as thousands of acres of previously irrigated land have not been farmed in recent years due to widespread irrigation well curtailments.

Rights to Renewable Water: Some of the water providers requesting reallocated storage space rely on non-tributary groundwater from the Denver Basin aquifer, which cannot be replenished with rainfall or snowmelt runoff. This non-renewable resource will be depleted over time and become much more expensive to extract in the future. Accordingly, many water providers have secured rights to surface water in the South Platte River and Plum Creek in order to decrease their dependence on nonrenewable aquifers. Many regional providers are looking for a storage solution to capture water, when available, under their water rights and store that water until needed. Additional storage in the joint flood control/multi-purpose pool at Chatfield Reservoir could provide a solution by allowing storage of renewable water, and partially alleviating the use of nonrenewable groundwater.

Potential Benefits of Reallocation: Several key benefits of the project include:

  • Storage for water supplies that makes use of an existing facility-construction of a new facility would not be necessary. New water yields would result from the storage of water.
  • The reservoir is located directly on the South Platte River, or “on-channel”, meaning that the cost of constructing a diversion and delivery system to another storage facility would not be necessary.
  • Chatfield is located at a relatively high elevation within the watershed, providing the opportunity to deliver water to some of the water providers by gravity flow which is more cost and energy efficient than having to pump water for delivery.
  • Because some providers already receive water from Chatfield, there is less need for the construction of new delivery systems.
  • Strategic releases of water from Chatfield could provide recreational and environmental benefits to the urban and downstream reaches of the South Platte River while providing water during low flow periods.
  • Those water providers upstream of Chatfield could capture their reusable return flows in the reservoir for subsequent reuse making greater use of the scarce resource.
  • Water providers could store additional water supplies for the agricultural community in the South Platte Basin, where thousands of acres of previously irrigated land has not been farmed in recent years due to widespread irrigation well curtailments.

Potential Impacts of Reallocation: Several potential impacts from the project include:

  • The reallocation of storage space in Chatfield Reservoir could increase the water level up to 12 feet when water is stored during non-flood conditions.
  • Water level fluctuations could increase both in magnitude and frequency. The operation of the reservoir and resulting water levels are based on a number of factors, including the water elevation at the time, flow conditions downstream, the priority of water rights of downstream providers, requests for release of stored water, precipitation and evaporation.
  • The project would require mitigation for environmental impacts (both on-site and off-site as necessary) and modification of existing recreation facilities around the reservoir that would be impacted/ inundated by higher reservoir levels.
  • A reallocation of storage in the reservoir would require a change in the operations of the reservoir and the construction and/or relocation of infrastructure, such as roads and facilities, in Chatfield State Park.
  • Several key recreational facilities at Chatfield State Park would need to be relocated/modified.
  • Water quality, fish and wildlife habitat, and wetlands may be impacted in both positive and negative ways
  • Socioeconomic resources may be impacted in both positive and negative ways

Next Steps: A final decision on the Chatfield Reservoir Storage Reallocation Study has not been made, and the information provided as a part of this article is intended only for informational purposes. The Corps will release Draft and Final versions of the FR/EIS for review before issuing a Record of Decision (ROD) on any action. The public will have the opportunity to review and comment on the Draft FR/EIS when it is released, perhaps in mid-2009. For additional information about the study, its sponsors, and contributing entities, please visit www.chatfieldstudy.org.