Eightmile Dam rebuild & restoration

The EIS will consider three spillway design alternatives and two construction options.

As required by the State Environmental Policy Act (SEPA,) the EIS will also evaluate a No Action Alternative. The No Action Alternative serves as the baseline condition, against which the action alternatives are compared and is intended to illustrate the most likely scenario if the project is not implemented.

Alternative 1: Narrow spillway with gates

This alternative includes replacement of the existing dam with an earthen embankment and reinforced concrete dam structure equipped with automated control gates over the primary spillway. Three 4-foot-high, 20-foot-long automatic level control gates would be installed on top of the primary spillway, which would have a hard crest elevation of 4,667 feet. The gates would allow IPID to control the water level within the top 4 feet of the lake. When additional water supply is needed, IPID would raise the gates in the late spring or early summer to raise the lake to elevation 4,671 feet prior to releasing the water in the late summer. The gates would automatically lower if the lake level gets too high to protect the dam and prevent overtopping. For example, if a storm occurs when the gates are up and the lake is full, the gates would automatically lower to pass peak flows generated by the storm. This design would allow for a narrow primary spillway (60 feet wide) and therefore a smaller dam footprint compared to the wide spillway alternative (described below).

During extreme storm events, the lake would continue to rise above the primary spillway. Two 10-foot wide intermediate spillways on either side of the primary spillway would provide 20 feet of additional spillway width at an elevation of 4,671.5 feet. A secondary spillway would be created in a low spot south of the main dam structure by hardening an existing channel. The secondary spillway would have a crest elevation of 4,673 feet. The spillways would provide capacity to pass the design storm event (a storm that has the probability of occurring once in one million years) while maintaining the freeboard in the lake required by DSO.

Water would be released from the lake through a new 30-inch diameter low-level outlet pipe/siphon. The low-level outlet pipe/siphon would extend from an inlet submerged in the lake approximately 150 feet west of the new dam structure to an outlet in the Eightmile Creek channel approximately 314 feet downstream of the new dam structure. This would allow the lake to be drawn down to a low water surface elevation of 4,636 feet during drought conditions, which would allow access to stored water without pumping. The low-level outlet pipe would be located entirely within the Special Warranty Deed Area. IPID would release water during the late summer to maintain the water supply available for authorized diversions and instream flows in Icicle Creek. Releases through the low-level outlet pipe would be controlled by an automated plug valve at the downstream end of the pipe. IPID would have the ability to adjust the valve remotely to release the flows needed to meet downstream water supply and instream flow needs.

The primary spillway gates and low-level outlet valve at the lake would be powered by batteries charged by a solar panel. Lake levels, gate and valve positions, and other controls would be monitored remotely, and the equipment would be operated via radio signal requiring an antenna, which would be located at the dam site, and a transmitter located on a ridge outside the Wilderness. The controls and monitoring equipment would be concealed as much as possible to protect the equipment and preserve, to the extent possible, the aesthetic values of the Alpine Lakes Wilderness Area.

Alternative 3: Narrow Spillway without Gates

As noted below in Section 3.4, a number of scoping comments were received that suggested that the EIS should include an alternative dam design that meets the existing spillway elevation of 4,667 feet and does not allow Eightmile Lake to be lower than its historic low levels. As a result, Alternative 3 has been developed and will also be evaluated in the EIS.

Under Alternative 3, the dam type and configuration would be almost identical to that of Alternative 1, having a narrow spillway and a concrete spillway apron, but with no mechanical gates. The mechanical gates that are included as part of Alternative 1 would allow IPID to store up to a maximum water surface elevation of 4,671 feet with the gates activated.

Alternative 3 would not have any gates and would only be designed to store water up to a maximum water surface elevation of 4,667 feet. This alternative would have the same footprint as Alternative 1. Because Alternative 3 would not have mechanical gates, the primary spillway would include one continuous 60-foot-wide primary spillway section with a crest elevation of 4,667 feet. The secondary spillway for Alternative 3 would be identical to that described for both Alternatives 1 and 2.

The maximum volume of water that could be stored for release by the dam would be less with Alternative 3 than for the other two action alternatives. Alternative 3 would not meet all of the applicant’s objectives because there would be less potential water storage available for release to ensure against drought conditions. This alternative would require pumping to access water storage greater than approximately 1,700 acre-feet.

Operation

In general, operation of Eightmile Dam would be as follows under either action alternative:

• The lake would be allowed to fill annually through mid-July each year.
• IPID would then open the valve on the low-level outlet to start releasing water, as needed to meet downstream needs.
• IPID would close the valve on the low-level outlet at the end of the irrigation season (fall timeframe).
• Similar to existing dam conditions, the lake level would typically continue to drop due to seepage through the soils under the dam until early fall storms begin to refill the lake.
• The lake would refill through the winter and spring.

Under either alternative, the drawdown would be limited to an elevation of 4,642 feet during non- drought years and 4,636 feet during declared drought conditions (roughly once every 5 years). In addition, there is a known natural leak in the earthen materials that compose the lower most portion of the dam. The maximum drawdown would release 2,000 acre-feet. IPID would regulate the lake so that the water elevation does not fall below the target elevation as of October 1 of each year, unless releases are needed after October 1 due to drought. The lake would only be lowered to the low-water surface elevation in drought years in late-September to early-October.

IPID would limit the release of water for non-district purposes to be consistent with water volumes determined through a decision-making process and resulting water management tool to be created by the IWG and approved by the co-conveners (Ecology and Chelan County). This tool will set the total water quantity to be released each year under the existing water right for non-district purposes and the minimum elevation of the lake. This decision support tool will also direct the instantaneous release amounts and timing of releases. IPID would reserve the right to lower the lake beyond such limits if needed for district purposes but not below 4,642 feet, unless a drought is declared by the state.

Under the narrow spillway alternative, the water surface elevation in the lake would typically be held at a maximum elevation 4,667 feet except for a few weeks during the late spring and early summer when IPID would raise the gates over the primary spillway to capture additional runoff and raise the lake to a maximum water elevation of 4,671 feet. IPID would typically raise the gates in May and begin to draw down the lake in July. The gates would be lowered once the lake level is drawn down below the bottom of the gates (4,667 feet). Under the narrow spillway alternative, if the gates are raised and the lake fills, the gates would automatically lower to prevent the lake level from rising above 4,671 feet. During a storm, the gates would lower to provide additional spillway capacity to pass peak storm flows.

Under the wide spillway alternative, the spillway would be passive, meaning that there would be no gate or other adjustable controls. The lake would flow over the primary spillway when the lake fills to an elevation above 4,671 feet.

Construction Methods – Applicable to All Action Alternatives

Construction of the proposed project would require the transport of construction equipment and materials into and out of the Alpine Lakes Wilderness Area. Construction would require substantial earthwork, including the excavation and placement of large rock. Completing the work would require heavy construction equipment, such as excavators or other earthmoving equipment. Initially, two potential construction methods were identified for mobilizing equipment and materials to and from the site: (1) helicopter transport, and (2) overland vehicle transport. These methods were proposed for construction, and a combination of methods was also possible. Non-motorized wilderness ground transport (i.e., pack equipment and materials in and out using humans and pack animals, with no use of motorized equipment) could be used to supplement either method for the lighter equipment and materials able to be transported in smaller quantities.

As a result of scoping comments and ongoing discussions, IPID has removed overland ground-based vehicle transport through the Wilderness as a possible construction method. IPID is exploring improvements to a short section of a closed Forest Service Road, but improvements to the road would stop short of the Wilderness. This short section of ground access would facilitate worker access, equipment and supply loading and drop off, and vehicle parking that is away from the trailhead. Access to this location would ease congestion at the trailhead and reduce the elevation and distance for hiking and packing to the dam site.

Because of the hazard the dam presents, it is possible that some emergency action could be required in the future to address the dam’s deficiencies if neither of the action alternatives is implemented. However, it is not possible to predict with certainty what that action or what its effects would be. Consequently, for purposes of the EIS analysis, it is assumed that the existing state of the dam and its operation remain unchanged.

• Legal Notice
• Scoping Notice
• News Release
• Alternatives Summary Table
• Alternatives Description
• Design Figures for the Alternatives
• Special Warranty Deed Parcels

Ecology is the lead agency under the State Environmental Policy Act (SEPA,) the law requiring agencies to assess the environmental effects of proposed actions prior to making decisions. We determined that the project could result in potentially significant adverse environmental impacts, and are therefore developing an Environmental Impact Statement (EIS) prior to identifying a preferred alternative for implementation.  Ecology's Dam Safety Office determined that the existing Eightmile Dam is a high risk facility and must be rebuilt to protect public safety and maintain reliable irrigation supply. As lead agency, we are responsible for ensuring that the EIS be developed in accordance with SEPA requirements. The proponent for the project is the Icicle and Peshastin Irrigation Districts, the owner of the project.  

We are leading the EIS preparation, working with an independent consultant team. The EIS team is reviewing materials prepared by Ecology, IPID and their technical consultants, the U.S. Forest Service, and other sources to build upon previous analyses. All information is being independently evaluated for accuracy, completeness, and relevance to this evaluation. Where information needs to be updated or expanded upon, the EIS team is working with us to determine the best approach to fill those information needs.

Scoping is the first step in the Environmental Impact Statement (EIS) process. The purpose of scoping is to receive input on the range, or “scope” of issues to be studied in the EIS. This process is required under the State Environmental Policy Act (SEPA).  Scoping was conducted in December 2020 & January 2021, with the scoping comment period closing on February 1, 2021.

All comments submitted will be considered carefully as the scope of the EIS is being developed. The EIS team, working with Ecology's SEPA Responsible Official, the person overseeing the process, will incorporate those comments that are within the scope of the evaluation for this project. Comments received that address issues outside of this project, or beyond the jurisdiction of Ecology, will not be addressed in the EIS. A Scoping Summary has been prepared that outlines the range of issues identified, and how these issues will be addressed in the EIS. Comments received that will not be addressed in the EIS are also outlined. Individual scoping comments are not addressed in the Scoping Summary, but major themes are described.

Scoping Summary Report

Ecology and Chelan County issued a programmatic EIS in 2019. It evaluated the Icicle Creek Water Resource Management Strategy. That programmatic EIS evaluated five programmatic strategies to improve instream flows, improve sustainability of the Leavenworth National Fish Hatchery, protect tribal and non-tribal fish harvest, improve domestic water supply and agricultural reliability, enhance Icicle Creek habitat, and comply with state and federal law including the Wilderness Act within the Icicle Creek Subbasin.  The culmination of that process was the adoption of the program called the Icicle Strategy, intended to provide a program of integrated long-term water resource management and habitat restoration actions. The Eightmile Dam Rebuild and Restoration Project is one of the first projects proposed to be implemented as part of the Icicle Creek Strategy, and is the first project-level EIS undertaken in this phased review process under SEPA. A project-level EIS looks at a specific project action, typically in greater detail than a programmatic EIS.

The Icicle Creek Work Group is composed of local, state, and federal agencies, tribes, irrigation and agricultural interests, municipal/domestic water managers, and environmental organizations. They are working together on a facilitated process to develop a plan to become drought resilient and climate responsive to meet the wide range of Icicle Creek subbasin water needs now and in the future. The Eightmile Dam EIS is a separate but parallel process from the Icicle Workgroup. The EIS is intended to inform Ecology as part of our decision making for approvals of the Eightmile Dam Rebuild and Restoration Project. Input from the Icicle Creek Work Group will be considered along with input from other stakeholders.  

Role of USFS
While Ecology is the lead SEPA agency for this EIS, the U.S. Forest Service (USFS) has provided input on relevant site information and participated in several public engagement opportunities. 

Ecology and the Okanogan-Wenatchee National Forest are jointly reviewing the proposed action to determine the responsibilities of each agency. The USFS will use agency regulations at 36 CFR 220.4(a) to determine which elements of the proposal require environmental analysis under the National Environmental Policy Act (NEPA,) the federal counterpart of SEPA, and will issue an independent NEPA decision for actions or approvals under USFS authority.

As authorized by the Council on Environmental Quality regulations, the USFS will cooperate with Washington on the state’s environmental analysis (40 CFR 1506.2(a)) and may use elements of the environmental review prepared under the State Environmental Policy Act (SEPA) process for any required NEPA analysis (40 CFR 1506.2(b)).

The USFS will continue to work with Ecology as the project moves through the environmental analysis process to ensure any responsibilities under the USFS authority will be met.

Tracking updates in the National Environmental Policy act (NEPA) process
The proposal presented by Ecology during Scoping is being refined through the SEPA process. Ecology is working with the USFS and the IPID to identify any actions which require USFS authorization. 

Once a finalized proposal is developed, the USFS will begin the appropriate NEPA analysis, including a post on their Schedule of Proposed Actions. Proposals subject to the USFS’s NEPA procedures will follow the agency’s applicable public involvement processes.

Lands were conveyed from the Icicle and Peshastin Irrigation Districts (IPID) to the U.S. government as National Forest System lands through a special warranty deed in 1990. Parcels and specific rights associated with the facilities, operations, maintenance, etc. at Eightmile Lake were retained by the IPID as described in the special warranty deed, and are therefore outside the jurisdiction of the U.S. Forest Service. As stated in the scoping documents, the alternatives currently under consideration do not extend outside the irrigation districts' deeded lands area, however, associated actions do extend outside the deeded area. 

Map displaying parcels

The No Action Alternative serves as the baseline condition against which the Action Alternatives are compared and is intended to illustrate the most likely scenario if the project is not implemented, consistent with SEPA guidance. Under the No Action Alternative, the dam would be left as is, with a primary spillway elevation of 4,667 feet, and would continue to operate in its current state and manner. This would leave the dam vulnerable to failure which could threaten human lives downstream, and create economic hardship for the IPID. Should a dam failure occur, residences, public infrastructure, and wilderness habitat could be damaged or destroyed. Ecology's Dam Safety Office (DSO) currently requires IPID to leave the low-level outlet gate open during the winter and early spring to reduce the risk of a dam failure. The operation of the dam in this manner is not consistent with DSO regulations, does not meet the DSO’s safety requirements for a high hazard dam, and would ultimately result in enforcement action by DSO. The No Action Alternative does not meet IPID objectives for water storage capacity for operational and irrigation water delivery, however, as noted above, this pathway outlines the most likely scenario if an action alternative is not implemented.

A hydrologic and hydraulic (H&H) study was done by IPID’s technical consultant, Anchor QEA, to assess the ability of the existing dam to pass storm flows. It was determined that the dam would not safely pass the required flow. During this process, Ecology’s Dam Safety Office (DSO) raised the safety rating of the dam to a high hazard level due to the human induced development downstream of the dam. This high hazard rating resulted in the need for the dam to meet higher storm thresholds.  

The dam alternatives will be designed to pass a 1 in one million storm event.  The lowest probability that DSO designs any dam is to a 1 in 500-year storm. The current configuration of the Eightmile Dam is estimated to pass approximately a 1 in 30-year storm.

The area impacted by the Jack Creek fire is above the existing and proposed lake water line, and will not be affected by dam operations. IPID is concerned about sediment entering the lake from the burned area but raising and lowering the lake will not affect the sedimentation rate from the burn scar.

While the IPID is responsible to pay for their proposed action and project construction, they are hoping to receive grant(s) to help defer the costs. The IPID has applied for WATERsmart federal grants, FEMA grants, and the Office of Columbia River grants.