USFWS BO 10_9_2020 01EWFW00-2018-F-1709.pdf

INTERIOR REGION 9

COLUMBIA–PACIFIC NORTHWEST

Idaho, Montana*, Oregon*, Washington

*PARTIAL

United States Department of the Interior

FISH AND WILDLIFE SERVICE

Eastern Washington Field Office 11103 E. Montgomery Dr.

Spokane Valley, Washington 99206

In Reply Refer To:

01EWFW00-2018-F-1709

X-Ref: 1-9-08-F-0013

Michael S. Erickson Chief, Environmental Compliance Section Corps of Engineers, Walla Walla District 201 North Third Avenue Walla Walla, Washington 99362-1876

Dear Mr. Erickson:

Subject: Mill Creek Project Operations and Maintenance Biological Opinion

This letter transmits the U.S. Fish and Wildlife Service's (Service) Biological Opinion on the proposed Mill Creek Flood Control Project Operations and Maintenance, located in Walla Walla County, Washington, and its effects on bull trout (Salvelinus confluentus) and critical habitat for the bull trout. Formal consultation on the proposed action was conducted in accordance with section 7 of the Endangered Species Act of 1973, as amended (16 U.S.C. 1531 et seq.) (Act).

Your September 14, 2018, request for formal consultation was received on September 18, 2018.

Please provide any comments on this biological opinion within 14 days of the date of this letter, or we will sign the document as is.

The enclosed Biological Opinion is based on information provided in the September 2018, Biological Assessment, telephone conversations, field investigations, emails, and other sources of information cited in the Biological Opinion. A complete record of this consultation is on file at the Service's Eastern Washington Field Office in Spokane, Washington. An electronic copy of this Biological Opinion will be available to the public approximately 14 days after it is finalized and signed. A list of Biological Opinions completed by the (Service) since October 1, 2017, can be found on the (Service) Environmental Conservation Online System (ECOS) website at https://ecos.fws.gov/ecp/report/biological-opinion.html.

https://ecos.fws.gov/ecp/report/biological-opinion.html.

Michael S. Erickson 2

The Corps of Engineers determined the proposed action would have no effect on the yellow-billed cuckoo (Coccyzus americanus). There is no requirement for concurrence by the Service on “no effect” determinations. Therefore, your determination rests with the action agency.

If you have any questions regarding the enclosed Biological Opinion, our response to your concurrence request(s), or our shared responsibilities under the Act, please contact Michelle Eames at (509) 393-5667, or michelle_eames@fws.gov.

Sincerely, Brad Thompson, State Supervisor Washington Fish and Wildlife Office

Enclosure(s) cc:

Corps, Walla Walla, WA (B. Tice) for

Endangered Species Act - Section 7 Consultation

BIOLOGICAL OPINION

U.S. Fish and Wildlife Service Reference:

01EWFW00-2018-F-1709

Mill Creek Flood Control Project Operations and Maintenance

Walla Walla County, Washington

Federal Action Agency:

Army Corps of Engineers, Walla Walla District

Consultation Conducted By:

U.S. Fish and Wildlife Service Washington Fish and Wildlife Office

Spokane, Washington

Brad Thompson, State Supervisor Date Washington Fish and Wildlife Office for

TABLE OF CONTENTS

ii

1 INTRODUCTION

2 CONSULTATION HISTORY

3 BIOLOGICAL OPINION

4 DESCRIPTION OF THE PROPOSED ACTION

4.1 Structural Components and Operations of the Mill Creek Flood Control Project

4.1.1 Mill Creek Diversion Dam, Low-Flow Outlet, and Spillway

4.1.2 Low-Flow Outlet

4.1.3 Mill Creek Diversion Dam Fish Ladder

4.1.4 Intake Canal and Intake Canal Headworks

4.1.5 Flow Regulation at Mill Creek Diversion Dam

4.1.6 Diversions for Fish and Wildlife Propagation and Recreation

4.1.7 Earthen Dike (Forebay Dike)

4.1.8 Debris Barriers

4.1.9 Bennington Lake and Storage Dam

4.1.10 Mill Creek Return Canal

4.1.11 Russell Creek Outlet Channel

4.1.12 Mill Creek Stabilized Channel

4.1.13 First Division Works

4.1.14 Second Division Works

4.1.15 Garrison Creek Fish Screen

4.1.16 Yellowhawk Weir and Fish Passage

4.1.17 Project Office, Rooks Park and Related Facilities

4.2 Project Maintenance and Future Construction

4.2.1 Mill Creek Diversion Dam and Associated Structures

4.2.2 Mill Creek Diversion Dam Spillway

4.2.3 Mill Creek Diversion Dam Low-Flow Outlet

4.2.4 Mill Creek Diversion Dam Fish Ladder

4.2.5 Bennington Lake and Dam

4.2.6 Mill Creek Stabilized Channel

4.2.7 First Division Works

4.2.8 Second Division Works

4.2.9 Project Office and Associated Structures

4.2.10 Rooks Park and Related Facilities

4.2.11 Summary of Conservation Measures

4.3 Action Area

5 ANALYTICAL FRAMEWORK FOR THE JEOPARDY AND ADVERSE

MODIFICATION DETERMINATIONS

5.1 Jeopardy Determination

5.2 Adverse Modification Determination

6 STATUS OF THE SPECIES: Bull Trout 7 STATUS OF designated CRITICAL HABITAT: Bull Trout 8 ENVIRONMENTAL BASELINE: Bull Trout and Designated Bull Trout Critical

Habitat iii

8.1 Current Condition of the Species and Critical Habitat in the Action Area

8.1.1 Walla Walla River Core Area

8.1.2 Factors Responsible for the Condition of the Species

8.1.3 Ongoing Consequences of Existing Structures

8.1.4 Status of Critical Habitat and Factors Responsible for the Condition of

Critical Habitat

8.2 Conservation Role of the Action Area

8.2.1 Walla Walla River Core Area and Mill Creek

8.3 Conservation Role of Critical Habitat

8.4 Climate Change

8.4.1 Climate Change Columbia River Basin

8.4.2 General Bull Trout Response to Anticipated Climate Change

8.4.3 Mid-Columbia Recovery Unit

9 EFFECTS OF THE ACTION: Bull Trout And Designated Bull Trout Critical Habitat. 62

9.1 Insignificant and Discountable Effects (Bull Trout and Designated Critical Habitat) 62

9.2 Exposure of Bull Trout

9.3 Adverse Effects to Bull Trout

9.3.1 Altered Stream Flows

9.3.2 Flood Diversions and Entrainment

9.3.4 Bennington Lake Maintenance and Flow Regulation

9.3.5 Fish Salvage and Disturbance of bull trout

9.3.6 Injury, Mortality, and Entrainment

9.3.7 Partial dewatering of sites or habitat impacts, water quality or water quantity

9.3.8 Vegetation Management

9.4 Effects to Critical Habitat

9.5 Summary of Effects

10 CUMULATIVE EFFECTS: Bull Trout and Designated Bull Trout Critical Habitat

10.1 Habitat Improvement

10.2 Chinook Salmon Reintroduction

10.3 Continued Development

10.4 Continued Use of Mill Creek Water for Municipal Water

10.5 Continued Use of Irrigation Water

11 INTEGRATION AND SYNTHESIS OF EFFECTS: Bull Trout and Designated Bull Trout Critical Habitat

11.1 Bull Trout

11.2 Critical Habitat

12 CONCLUSION: Bull Trout and Designated Bull Trout Critical Habitat

13 INCIDENTAL TAKE STATEMENT

14 AMOUNT OR EXTENT OF TAKE

15 EFFECT OF THE TAKE

16 REASONABLE AND PRUDENT MEASURES

17 TERMS AND CONDITIONS

iv

18 CONSERVATION RECOMMENDATIONS

19 REINITIATION NOTICE

20 LITERATURE CITED

APPENDICES

Appendix A Status of the Species: Bull Trout Appendix B Status of Designated Critical Habitat: Bull Trout

FIGURES

Figure 1. Mill Creek Flood Control Project Figure 2. Overview of the Project footprint Figure 3. Mill Creek Operation and Maintenance Federal Footprint Figure 4. Approximate Action Area Boundary Figure 5. Mill Creek/Yellowhawk Creek PIT Tag Detections Figure 6. Mean Monthly Flows Calculated from Historic (representative) conditions Mill

Creek (cfs) Figure 7. Number of Flood Diversion Days by month between 1945 and 2017, overlaying spring Chinook salmon, summer steelhead, and bull trout abundance

TABLES

Table 1. Maintenance Requiring the Forebay to Be Lowered Table 2. Bull trout spawning survey summary, redd count (number of times surveyed), for

Mill Creek, 1990-2017 Table 3. Normal precipitation for Walla Walla County, Washington Table 4. Summary of baseline structure effects and bull trout passage impairment Table 5. Baseline Section 7 Consultations Table 6. Actions with Discountable or Insignificant Effects Table 7. The frequency and duration of flood events resulting in unscreened diversions of

Mill Creek into Bennington Lake since 1945 Table 8. Mill Creek Flow and Average Recurrence Interval Flow (cfs) Table 9. The number of days per month that the low-flow outlet and the fish ladder were closed when flows exceeded 400 cfs preventing any upstream fish passage since ladder was installation in 1984. In almost all cases, the days are consecutive

Table 10. Progress on RUIP actions under the Control of the Corps v

ACRONYMS AND ABBREVIATIONS

2018 BA Biological Assessment from 2018 on Mill Creek O&M Project BA Biological Assessment BMP best management practices CFR Code of Federal Regulations cfs cubic feet per second Corps U.S. Army Corps of Engineers CTUIR Confederated Tribes of the Umatilla Indian Reservation cy cubic yards DAM Destruction or Adverse Modification [of critical habitat] ESA Endangered Species Act of 1973, as amended (16 U.S.C. l531 et seq.)

FERC Federal Energy Regulatory Commission FMO Foraging, Migration and Overwintering FPP Fish Passage Plan FR Federal Register FY Fiscal Year MCFCP Mill Creek Flood Control Project MCFCZD Mill Creek Flood Control Zone District MCRU Mid-Columbia Recovery Unit [for Bull trout] MCWG Mill Creek Working Group MOU Memorandum of Understanding NMFS National Marine Fisheries Service Opinion Biological Opinion PBF Physical or Biological Features PCE Primary Constituent Element PIT Tag Passive Integrated Transponder Tag Project Mill Creek Flood Control Operations and Maintenance Project RM River Mile RUIP Recovery Unit Implementation Plan SEIS Supplemental Environmental Impact Statement SRSRB Snake River Salmon Recovery Board TSS Total Suspended Solids USFS U.S. Forest Service USFWS U.S. Fish and Wildlife Service USGS U.S. Geological Survey WDFW Washington Department of Fish and Wildlife WDOE Washington State Department of Ecology

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1 INTRODUCTION

This document represents the U.S. Fish and Wildlife Service's (USFWS) Biological Opinion (Opinion) based on our review of the proposed Mill Creek Flood Control Operation and Maintenance Project (Project) located in Walla Walla County, Washington, and its effects on bull trout and designated critical habitat for the bull trout, in accordance with section 7 of the Endangered Species Act of 1973, as amended (16 U.S.C. 1531 et seq.) (ESA). Your September 14, 2018, request for consultation with an attached Biological Assessment (2018 BA) was received on September 18, 2018.

This Opinion is based on information provided in the 2018 BA, emails, telephone conversations, field investigations, and other sources of information as detailed below. A complete record of this consultation is on file at the Eastern Washington Field Office in Spokane, Washington.

2 CONSULTATION HISTORY

The following is a summary of important events associated with this consultation:

• The Walla Walla District, U.S. Army Corps of Engineers (Corps) initially requested formal consultation on an earlier proposed action with a letter dated June 16, 2015, received in our office on June 23, 2015. The request included a biological assessment.

• On January 7, 2016, the USFWS sent a letter to the Corps noting that we had not received sufficient information to initiate consultation, and both agencies agreed to coordinate on information needs, description of the proposed action, and participate in conference calls and meetings as needed during the consultation process.

• The USFWS completed a biological opinion on a similar proposed action in 2007. Since then the Corps developed a supplemental Environmental Impact Statement (SEIS) on the project, and modified the proposed action. On January 20, 2018, the Corps and USFWS agreed to pause the consultation process to allow the Corps to complete the SEIS and revise the proposed action. The Draft SEIS was dated May, 2018.

• Your revised September 14, 2018, request for consultation with an attached Biological Assessment (2018 BA) was received in our office on September 18, 2018.

• The USFWS requested additional information in a letter dated November 27, 2018.

• Additional information was received via email on December 13, 2018. Formal consultation was initiated on December 13, 2018, although the Corps and the USFWS agreed that we would not meet the 135-day timeline [May 2, 2019] due to the complexity of the Project and continued need to work back and forth as we proceed.

• On April 9, 2019, the USFWS received requests for formal consultation, with attached biological assessments, on two separate projects which are interrelated to the Project: the Mill Creek Diversion Dam Grouted Riprap Repair Project in Mill Creek, and the Mill Creek Division Works Fish Ladder Replacement Project. After review of these proposed projects, the USFWS determined they should stay together in one large action; therefore we proceeded with the original consultation request for the Project.

• On January 30, 2020, the Corps emailed a copy of a letter to the USFWS, dated January 28, 2020. The letter changed the diversion trigger to 1,700 cubic feet per second (cfs) year round, rather than the variable criteria by months as expected in the 2018 BA. The Corps also provided information on the estimated frequency of diversions with the new criteria, and potential numbers of bull trout that may be entrained into Bennington Lake during a flood (Tice, in litt. January 31, 2020).

In the 2018 BA, the Corps determined there would be no effect to yellow-billed cuckoo (Coccyzus americanus). There is no requirement for USFWS concurrence on "no effect" determinations. Therefore, this determination rests with the action agency.

3 BIOLOGICAL OPINION

4 DESCRIPTION OF THE PROPOSED ACTION

A federal action means all activities or programs of any kind authorized, funded, or carried out, in whole or in part, by federal agencies in the United States or upon the high seas (50 CFR 402.02).

The Project is part of the Mill Creek Flood Control Project (MCFCP) and is located near and in the city of Walla Walla, Washington, in Walla Walla County. The MCFCP is managed both by the Mill Creek Flood Control Zone District (MCFCZD) and the Corps. The structural components of the MCFCP are divided into two sections based on ownership and management responsibility: (1) RM 4.8 to 10.4 is locally-managed by the local MCFCZD and; (2) RM 10.4 to approximately 11.5 is the federally-owned and managed portion. The locally managed portion consists of approximately four miles of riprapped levees and channel spanning weirs, and two miles of concrete channel. The Proposed Action is the Corps’ operations and maintenance of the Mill Creek Flood Control Project.

The Corps-managed Project extends from approximately River Mile (RM) 10.4 upstream to roughly 0.3 mile upstream of the Mill Creek Diversion Dam (RM 11.5) of Mill Creek. The federal portion consists of two dams, the Division Works Dam at RM 10.4 and Mill Creek Diversion Dam at approximately RM 11.5, approximately 1.1 miles of stabilized channel between the two dams and 556 acres of nearby land and open water in Bennington Reservoir.

The action includes all of the current operations and maintenance actions included in Appendix A of the 2018 BA, and new operations and maintenance actions included in Appendix B of the

2018 BA.

The actions considered under this consultation include: (1) operation of the Project (Figure 1) under its current structural configuration, (2) short-term and long-term actions that the Corps will implement beginning in fiscal year (FY) 2020 regarding removal of passage barriers, (3) changes in the operation after structural modifications are completed and, (4) current and future maintenance of the Project (Corps 2018a; 2018b; 2018c).

Operations generally consist of the opening and closing of various gates and the two fish ladders and vary based on three separate flow regimes. The first with flows up to 400 cfs, the second when flows are above 400 cfs but less than 1,700 cfs, and the third when flows exceed 1,700 cfs, which are considered floods. This Opinion does not describe or analyze emergency conditions other than flood operations.

Figure 1. Mill Creek Flood Control Project Operations

The Project is operated year round. While most maintenance activities would occur during the warmer months, some maintenance actions also occur year round (2018 BA p.6). Most planned in-water work activities would occur during the normal in-water work window between July 15 and September 15 unless coordination with the National Marine Fisheries Service (NMFS), the

USFWS, and the Washington Department of Fish and Wildlife (WDFW) has occurred. An exception will include if the in-water work timing is expected to be extended beyond the normal dates for the Division Works Dam Fish ladder, described later in this document.

During the consultation process, the USFWS, Corps, and NMFS had many discussions about the proposed action, and determined the following timing and duration for components of the Action. Structural changes are subject to availability of funding, however the USFWS assumes the following timing will be met:

• The Corps will be replacing the fish ladder at the Division Works during the summer of 2020.

• After discussion with the Corps, the USFWS assumes the low flow channel components of the action will begin by 2025, and will take five years to complete.

• A new fish ladder at Mill Creek Diversion Dam will be designed by FY 2023 and constructed by FY 2025, (Tice and Driscoll, in litt. October 30, 2019).

• There would also be a change in the flood diversion trigger from the current 1,400 cfs year-round to 1,700 cfs. The operations and maintenance of the Project also includes vegetation management.

• The duration of the entire proposed action was not stated in the 2018 biological assessment. After discussion with the USFWS, the Corps determined the duration of the proposed action is 10 years from the time the Opinion is issued (Tice in litt. September 22, 2020). Because the Corps is committing to structural changes in the project footprint, it is likely that flow management will change in the future after structures are completed to better ensure continued fish passage at as many flows as possible. At 10 years, the projects should be completed and operations will be re-evaluated.

Below we provide a detailed description of the numerous structures and their operations, followed by a discussion of maintenance and construction. The following sections first discuss structural components and flow operations (sections 4.1 and 4.2), then project maintenance and structural changes (section 4.3). Generally the discussion is arranged starting from upstream and moving to downstream.

4.1 Structural Components and Operations of the Mill Creek Flood Control Project

During flood events the Corps operates the Project to store excess flood water that would otherwise cause flood damage to the City of Walla Walla. By reducing flood flows through the city of Walla Walla and downstream areas, the risk of property damage and loss of life caused by flooding is reduced. The Corps operates the Project during such events according to the Water Control Manual for the Mill Creek Project (2018 BA).

4.1.1 Mill Creek Diversion Dam, Low-Flow Outlet, and Spillway

The Mill Creek Diversion Dam is located on Mill Creek at RM 11.4. Structures associated with the Mill Creek Diversion Dam include the low-flow outlet, fish ladder, spillway, intake canal headworks, diversion dam dike, and debris barriers. The purpose of the Mill Creek Diversion Dam is to raise the water level behind the dam during floods to make it possible to divert floodwaters from Mill Creek through the intake canal headworks and intake canal and into the storage reservoir, Bennington Lake. Water passes through the dam via the low-flow outlet and fish ladder when flows are less than 400 cfs (the maximum capacity of the low-flow outlet and fish ladder). For flows above 400 cfs but less than 1,700 cfs, the outlet is closed and water passes over the spillway. Flows above 1,700 cfs are considered flood levels and the fish ladder and low-flow outlet remain closed while flows are divided between spilling over the dam and being diverted unscreened into Bennington Lake.

4.1.2 Low-Flow Outlet

The low-flow outlet is located on the south side of the spillway and passes water through the Mill Creek Diversion Dam into the stabilized channel. The amount of flow passing through the outlet is controlled by a 6-foot-wide by 8-foot-high automatic gate. A small stilling basin (19.5 feet wide, 4 feet long, and 2 feet deep) is located downstream of the low-flow outlet channel and fish ladder.

4.1.3 Mill Creek Diversion Dam Fish Ladder

The existing Mill Creek Diversion Dam fish ladder was constructed in 1982. It is located at the south end of the dam, adjacent to, and south of, the low-flow outlet. The ladder is designed to operate fully open under normal conditions. The ladder is covered by a walkway grating. The grating creates shade over about 50 percent of the ladder. A series of large concrete blocks were added on the north side of the ladder entrance to concentrate the flow to help fish find the fish ladder entrance and create deeper water for improved passage.

The fish ladder is designed to operate when Mill Creek flows are less than 400 cfs although it becomes a partial barrier at approximately 200 cfs and higher. The operation range is maintained automatically by the low-flow outlet gate during normal flows of less than 400 cfs. The ladder is operated year round, except during maintenance or when flow is expected to exceed 400 cfs.

During periods of flows greater than 400 cfs the ladder is manually closed, and would not be passable even if left open. If flow is projected to reach or exceed 400 cfs for longer than 24 hours, the low-flow outlet is also closed, the forebay fills, and water flows over the spillway.

When Mill Creek flow is below 400 cfs, most of the water is passed through the low-flow outlet.

Flow through the ladder varies from about 20 to 42 cfs (fish ladder design capacity (Corps 2007 p.14). As flow recedes, as much water as possible is passed through the fish ladder, and the remainder is passed through, or leaks around, the low-flow outlet gate. Once the risk of needing Bennington Lake for flood storage has passed, some screened water (diverted through fish screens) is also diverted to fill Bennington Lake to benefit fish and wildlife and recreation.

The Corps expects to complete design in 2023 and then construct the new ladder at Mill Creek Diversion Dam in 2025. Construction details of the new ladder are not yet known, therefore the Corps may need to conduct a site-specific consultation on the construction. After construction, operations will be adjusted to manage flows for effective fish passage at the highest range of flows (Tice in litt. April 21, 2020).

4.1.4 Intake Canal and Intake Canal Headworks

The intake canal headworks consists of four intakes, each with an 8-foot-high by 18-foot-wide gate. The total design capacity of all the gates is 7,000 cfs. The gates are controlled manually, but are powered by electric motors. If power is lost, they can be operated by other means, including by hand.

The intake canal is 1,800 feet long and lined with concrete. The typical channel cross-section is trapezoidal with a 60-foot bottom width and steep side slopes. The last 300 feet of the channel expands to a width of 225 feet before entering a soil and rock channel leading to Bennington Lake. The expanded section has a series of concrete diffuser blocks that dissipate energy to reduce erosion before the water enters the soil and rock channel.

The headworks and intake canal, in combination with the Mill Creek Diversion Dam, work together to divert water from Mill Creek into Bennington Lake.

4.1.5 Flow Regulation at Mill Creek Diversion Dam

Under normal flow (below 400 cfs), the Mill Creek Diversion Dam is operated to maintain enough water depth in the forebay to operate the fish ladder. As flow fluctuates, a sensor monitors the water level and adjusts the outlet gate automatically. When flow higher than 400 cfs is forecast to last more than 24 hours, the low-flow outlet and fish ladder gates are manually closed, the forebay fills, and all flow passes over the spillway (and if conditions warrant, some of the unscreened flow is diverted to Bennington Lake). The spillway has a short downstream concrete stilling basin with concrete energy dissipaters, and it discharges into the Mill Creek Stabilized Channel.

When Mill Creek flow reaches 1,000 cfs, the Corps initiates a decision process in accordance with the Corps Water Control Manual for the Project to determine whether to divert water to Bennington Lake and how much to divert based on weather, snowpack, and other conditions.

Although this decision process is initiated at 1,000 cfs, the Corps does not divert floodwater until the flow in Mill Creek at Walla Walla stream gage reaches 1,700 cfs (2018 BA).

The National Weather Service River Forecast Center provides a river flow forecast for the Walla Walla River at Touchet, Washington, RM 18.2 and near Kooskooskie, Washington, at RM 21.2.

The Corps uses the forecasts to predict the potential Mill Creek inflow. Due to the high slope of the stream and watershed, it is very difficult to accurately forecast inflow on Mill Creek. The decision process to divert unscreened flows into Bennington Lake is initiated when the flow at the USGS Gage 14015000 (RM 10.4), Mill Creek at Walla Walla, Washington, reaches 1,000 cfs, while a diversion will not actually occur until Mill Creek flow is anticipated to exceed 1,700 cfs. This 1,700 cfs flow is referred to as the diversion trigger and has varied over the years (from 3,500 cfs to 1,100 cfs). The remaining flow downstream of the diversion dam after flood water is diverted is called regulated flow. Subsequent to the initial diversion trigger, the Water Control Manual indicates to increase the amount being diverted as the flood progresses and Bennington Lake fills. In other words, the diversion trigger is only the starting point for diversions. The amount being diverted changes in response to actual conditions and is designed to provide the maximum flood damage reduction possible.

Flows above 1,700 cfs are diverted unscreened into Bennington Lake until the flood event is over or until the lake elevation reaches established limits. The lake has no egress for fish that are diverted in the high flows to return to Mill Creek. When the lake level exceeds the established limits, the regulated flow down Mill Creek can be increased up to a maximum of 3,500 cfs as the flood flow increases. An initial regulated flow of 1,700 cfs passing the Project office gage is used to limit flows in the lower, natural channel reach (below Gose Street Bridge) to approximately natural channel capacity and provide flood risk reduction for the more frequently expected floods. The Corps’ objective is to keep the regulated flow between 1,700 cfs and 3,500 cfs, to minimize flood damage to the extent possible as described in the Water Control Manual

(2018 BA).

4.1.6 Diversions for Fish and Wildlife Propagation and Recreation

After the threat of flooding has passed, but prior to the end of the WDFW water right on June 15, the Corps, on behalf of the WDFW, may divert up to 30 cfs from Mill Creek to fill Bennington Lake for fishing and recreation. The WDFW stocks Bennington Lake with trout and other fish.

Flow up to 30 cfs is passed down the intake canal until the lake reaches an elevation of 1,205 feet. This typically takes 10 to 15 days to complete. Then only enough water is diverted to maintain the Bennington Lake level at 1,205 feet until flow in the creek declines to 40 cfs.

Recreation diversions are only necessary if the lake has not already been filled by winter and early spring flood flow.

Recreation diversions pass through two rotating drum fish screens installed in front of the easternmost intake gate. The screens were installed in 2001 by the Corps and WDFW to avoid entraining ESA-listed fish into Bennington Lake when it is filled for public recreation. The screens are each approximately 8 feet wide and 7.5 feet in diameter and can pass up to 30 cfs.

They are designed to operate with a water depth from about 4.9 feet, to 6.4 feet (65 percent to 85 percent of the height of the screen). The screens can only operate when the forebay is at or below 1,255.5 feet and flows in Mill Creek are less than 400 cfs. Operating the screens with a water depth greater than 6.4 feet (over 85 percent of the height) would damage the screen motors and could cause fish to be pinned against the screens and then carried over the screen.

Removable trash racks are located about 10 feet upstream of the rotating drum fish screens.

These closely spaced metal bars prevent debris from damaging the screens. Stoplogs downstream of the screens ensure proper flow through the screens. An overhead crane is used to raise the screens for safe storage when flow rises above 400 cfs and for maintenance. The trash racks and stoplogs are also removed when flow rises above 400 cfs.

4.1.7 Earthen Dike (Forebay Dike)

The north earthen dike forms the northern boundary of the Mill Creek Diversion Dam and confines water to the main channel. The dike extends upstream 2,200 feet from the north end of the spillway. The top of this embankment varies in elevation from 9 to 19 feet higher than the spillway.

4.1.8 Debris Barriers

Two debris barriers in the forebay help keep debris from damaging or plugging Project components. The first debris barrier is a 550-foot-long, steel crib and cable structure that captures many of the floating logs. It helps limit large debris from clogging the gates and plugging the rotating drum fish screens in the intake canal headworks. There is also a measurement gage on this structure that shows the water surface elevation.

Although the first debris barrier is designed to catch the majority of the large floating debris, it is important that both barriers be monitored closely during a major flood event. If a significant amount of debris is bypassing the barriers, then the Mill Creek Diversion Dam forebay area and downstream structures (e.g., first division works, covered channel sections, and bridges) need to be checked more frequently during the flood event.

4.1.9 Bennington Lake and Storage Dam

Bennington Lake is an off-channel, flood storage reservoir with a surface area ranging from about 20 to over 200 acres, and a depth ranging from about 10 to 80 feet. The surface area is approximately 52 acres at the recreational lake elevation of 1,205 feet. The storage dam was constructed with compacted soil. It is 3,200 feet long with a 20-foot width at the crest and 800-foot width at the base, and it is 125 feet high with a crest elevation of 1,270 feet. While the lake has several ways of releasing water, it does not provide any mechanism for fish that are diverted with the unscreened high flows to exit the lake and continue their anadromous life history behaviors.

The intake tower is located in the lake near the storage dam. It is used to release water from Bennington Lake into the Mill Creek Return Canal and the Russell Creek Outlet Channel to evacuate the reservoir and make space available for a future flood event. Evaporation and seepage can cause the lake to fall to an elevation of about 1,185 feet (20 surface acres) by the end of summer. Each fall, the lake is drained via the Russell Creek Canal to enable inspection and maintenance of the intake tower.

As described above, when Mill Creek flow exceeds 1,700 cfs, floodwaters are diverted to Bennington Lake to the maximum allowable level. If the lake rises above elevation 1,212 feet, water can be released from the lake through the intake tower to the Mill Creek Return Canal. If the lake elevation does not reach, or falls below 1,212 feet, water is released through the Russell Creek Outlet Channel. In an emergency situation, above elevation 1,212 feet, both the return canal and the outlet channel could be used at the same time, and if the lake elevation is higher than the Mill Creek elevation at the diversion dam, water could be drained back to Mill Creek using the intake gates and spillway. The lake should not be maintained above 1,235 feet for more than 15 days to limit high water pressures at the toe of the dam. Water must be rapidly released to below elevation 1,235 feet, even if some downstream flood damage could occur.

4.1.10 Mill Creek Return Canal

The Mill Creek Return Canal is about 1-mile long and includes various sections of shotcrete-lined open channel, corrugated metal pipe, and unlined (earthen) open channel. The canal is designed to carry a flow of up to 190 cfs. Water discharges from the canal into Mill Creek just upstream of the first division works and Project office. The canal is used to return water to Mill Creek in a controlled manner when releasing water from Bennington Lake after flood diversions have filled the lake to an elevation above 1,212 feet. This occurs about every 5 to 10 years on average; it was used in 1996, 1997, 2019, and 2020 after flood events.

4.1.11 Russell Creek Outlet Channel

The Russell Creek Outlet Channel is a 7,300-foot-long, concrete-lined, open channel, with a design discharge capacity of 250 cfs. The channel discharges into Russell Creek, a tributary of Yellowhawk Creek, about 1.25 miles from the Mill Creek Storage Dam. The outlet channel is used to drain the lake in a controlled manner for annual maintenance and to send water to Russell Creek after flood diversions have filled Bennington Lake to an elevation above 1,212 feet. The Russell Creek Outlet Channel was used in 1996, 2019, and 2020 to release floodwaters from Bennington Lake.

4.1.12 Mill Creek Stabilized Channel

The purpose of the Mill Creek stabilized channel is to protect creek banks and increase the capacity of Mill Creek to pass high flow (design capacity is 3,500 cfs). The portion of the stabilized channel operated and maintained by the Corps begins at the Mill Creek Diversion Dam and extends for about one mile to the boundary of the federal footprint, just downstream of the first division works. This area also includes the north and south levees lining the channel.

The Project portion of the stabilized channel has a trapezoidal cross section, with a varying bottom width and banks with a 1-foot vertical rise for every 2-foot horizontal side slope. Levees on both sides of the channel have minimum top widths of 13 feet and have large, angular riprap on the streamside slopes. The riprap covers a layer of wire-bound rock.

Levee vegetation consists primarily of grass and small shrubs. The channel bottom width transitions from 250 feet at the Mill Creek Diversion Dam to 120 feet just a few hundred feet downstream of the dam. The 120-foot channel width continues to the first division works where it transitions to 65 feet wide. Concrete-capped stabilizing weirs spanning the entire channel width, spaced 60 feet apart, control the slope of the channel. The Project portion of the stabilized channel contains 84 weirs; 79 between Mill Creek Diversion Dam and the first division works, and five downstream of the first division works.

Water depth downstream of the stabilizing weirs varies from about 1 foot to up to 5 feet in some areas. Several large rocks were placed in the channel for fish habitat in 1986 and replaced in 1997. Prototype low-flow weirs were installed in three of the channel-spanning weirs in 2012 to test the concept. These low-flow weirs are intended to improve fish passage conditions during low-flow periods.

The north levee runs along the north bank (right bank looking downstream) of the Mill Creek stabilized channel between the Mill Creek Diversion Dam and the Division Works Dam. The levee is surfaced with a paved pedestrian/bike trail. There is a covered picnic shelter adjacent to the Division Works Dam. The south levee runs along the south bank (left bank looking downstream) of the stabilized channel between the Mill Creek Diversion dam and the Division Works Dam. The Corps owns a narrow strip of land along the south side of the channel that extends 1,350 feet from the toe of the levee just upstream of the Mill Creek Return Canal to an area upstream (about river mile 11). Lands south of the Corps boundary in this area are privately owned. Near the Project office, the levee contains an area with a parking area.

The Mill Creek stabilized channel and levees are static structures. Operation is dependent on natural stream flow and operation of the Mill Creek Diversion Dam and first division works.

Water depth over the weirs within the Project area ranges from about 6 feet during flood events to less than 1-inch deep during the summer. Situations that could threaten the function of the Mill Creek stabilized channel include bank failure, debris restrictions, and excessive flow overtopping the levees. The levees are regularly inspected for seepage or damage, and during high-flow events levees are inspected at least daily.

4.1.13 First Division Works

The first division works is a water control structure on Mill Creek, located just north of the Project office at river mile 10.5 where Mill Creek diverges into the Yellowhawk/Garrison canal.

The first division works structure was originally constructed for flood risk management purposes. As originally designed, 1,400 cfs of flood water could be diverted from Mill Creek into Yellowhawk Creek (900 cfs) and Garrison Creek (500 cfs). In 1964, the Project’s Water Control Manual was revised to prohibit further use of Yellowhawk or Garrison Creeks for flood risk management purposes because of development and associated encroachments along these creeks.

Yellowhawk and Garrison Creeks now have a flow capacity of about 60 cfs and 10 cfs, respectively. Currently, the first division works structure is used for Washington State Department of Ecology (WDOE) directed diversion of non-flood flow from Mill Creek to Yellowhawk and Garrison Creeks for non-flood purposes, including improving conditions for fish. The first division works includes a dam (Division Works Dam) with four vertical lift gates, a fish ladder on Mill Creek, and a headworks to the Yellowhawk/Garrison canal with a fish passage slot on Yellowhawk Creek. The components of the first division works and their operations are described in the following paragraphs.

First Division Works Dam

Each vertical lift gate on the Division Works Dam is 25 feet wide by 2 feet high. When the gates are closed (lowered), they create a 2-foot-high weir that raises the water level to divert water through the Yellowhawk/Garrison canal headworks, while excess flow passes over the lift gates and through the fish ladder. The gates can be raised above the Division Works Dam’s bridge deck during flood operations. The opening created by all the gates being fully raised is 96 feet wide by 6 feet high.

The headworks is a concrete structure with three intakes, located just south of the vertical lift gates. The center intake includes a 14-foot-wide by 6-foot-high gate. A concrete bulkhead completely seals off the left intake, which was once used to help pass floodwaters into the Yellowhawk/Garrison canal. The right intake is also covered by a bulkhead with the exception of an 18-inch-wide slot to allow for fish passage. The bottom of the slot is at the same elevation as the stream bottom. A water depth of 1 to 3 feet at the slot creates corresponding water velocities ranging from about 8 to 14 feet per second.

Division Works Dam Fish Ladder

A fish ladder was constructed at the first division works on the right (north) bank of Mill Creek in 1982. A slide gate at the exit provides an open slot for optimal fish passage. Design capacity was 15 cfs, but a later analysis (2015 BA) indicated that, with operational adjustments, the ladder can pass as little as 3.8 cfs during low-flow and 20.7 cfs during high flow of 400 cfs or more.

The ladder provides upstream fish passage when all four vertical lift gates are in the fully lowered position for diverting water into the Yellowhawk/Garrison canal.

Yellowhawk/Garrison Canal

The Yellowhawk/Garrison canal is approximately 500 feet long and 30 feet wide and extends from the first division works to second division works, where it splits into Yellowhawk and Garrison Creeks. The banks of the canal are well vegetated, and a footbridge crosses the creek between the parking lot and the Project office. The canal flows through two culverts beneath Reservoir Road before terminating at the second division works.

First Division Works Operations

As described below, when flow is below 400 cfs, WDOE has the responsibility for flow regulation, and the WDOE watermaster directs the amount of water diverted into the Yellowhawk/Garrison canal in order to provide flow for fish and fish habitat. During flood events, the Corps operates the First Division Works to minimize flooding downstream areas along Yellowhawk and Garrison Creeks.

When Mill Creek flow is below 400 cfs, the Corps operates the First Division Works according to a Memorandum of Understanding between the Corps and WDOE (or as requested by WDOE) to maintain adequate flow for fish and fish habitat in Yellowhawk Creek. The Division Works Dam vertical lift gates are lowered (closed) to fill the forebay, and the headworks is adjusted to allow approximately 30 cfs into the Yellowhawk/Garrison canal. Further adjustments are made to maintain a minimum of 0.9 foot (~ 25 cfs) in Yellowhawk Creek. Any remaining flow in Mill Creek is passed down Mill Creek through the Division Works Dam, with 10 cfs maintained in the fish ladder.

When Mill Creek flow is between 40 cfs and 70 cfs, the Corps does not make any gate changes at the First Division Works that would cause flow to change more than 5 cfs. When Mill Creek flow is between 10 cfs and 40 cfs, fish downstream of the Division Works Dam may become trapped or are in danger because flows are too low and temperatures are too high. In an attempt to address this concern, the Corps manages flow (rapid, temporary flow changes), as outlined in the Fish Passage Plan (Corps 2007). The Yellowhawk/Garrison headworks is operated to decrease Mill Creek flow by 5 cfs in the morning and increase flow by 5 cfs in the evening. This fluctuation is intended to provide a signal to fish downstream of the Division Works Dam to move from that area before it becomes too warm for them.

When flow in Mill Creek downstream of the First Division Works Dam falls below 10 cfs, the 5-cfs fluctuation is discontinued, and a 6-inch low-flow restrictor plate is installed in the fish ladder exit. All flow is directed to the fish ladder in an attempt to improve passage at that location. During the summer low-flow season, however, the slide gate at the exit is sometimes used to partially close off the ladder to divert more water to the Yellowhawk/Garrison canal, and little to no water down Mill Creek. The MCFCZD (not the Corps) is responsible for operations and maintenance of the lower 6 miles of the Mill Creek stabilized channel, from the Project boundary to Gose Street.

Mill Creek Flow from 400 to 1,700 cfs

Although flooding officially exists at the Project when natural flow in Mill Creek exceeds 1,700 cfs, flood risk management operations actually begin when Mill Creek flow exceeds 400 cfs.

When flow exceeds 400 cfs, the Corps raises (opens) the two center vertical lift gates of the Division Works Dam and maintains flow to Yellowhawk Creek at 0.9 foot (~25 cfs). When flow on Mill Creek exceeds 1,000 cfs, the Corps opens all four vertical lift gates of the Division Works Dam and continues maintaining flow into Yellowhawk Creek at 0.9 foot (~25 cfs).

Mill Creek Flow above 1,700 cfs

When flow in Mill Creek exceeds 1,700 cfs, the Corps assumes control of all flow on Mill Creek, Yellowhawk Creek, and Garrison Creek for flood risk management purposes. The Corps maintains flow to Yellowhawk Creek at 0.9 foot, unless a lower flow is determined necessary due to flooding impacts downstream.

4.1.14 Second Division Works

The second division works is located at the downstream end of the Yellowhawk/Garrison canal, about 500 feet from the First Division Works and Mill Creek. It consists of a control structure that divides the flow between Yellowhawk and Garrison Creeks.

4.1.15 Garrison Creek Fish Screen

After determining Garrison Creek did not provide adequate passage for migratory fish, a stainless steel fish screen was added to the Garrison Creek intake in 2009, based on a cooperative partnership between several agencies (the Walla Walla County Conservation District, Bonneville Power Administration, WDFW, the Washington Salmon Recovery Funding Board, the Confederated Tribes of the Umatilla Indian Reservation (CTUIR), and the Corps (2015 BA p.53). The screen was designed with an appropriate flow velocity through the screen to prevent small fish from being pinned against the screen. The screen includes an automatic cleaning system with a sweeper and an air bubbler to reduce debris and ice build-up.

The fish screen cleaner is operated automatically by sensing water levels on the upstream and downstream sides of the screen. When the cleaner senses a difference of 2 or more inches, the sweeper cycles automatically across the screen. The screen air bubbler also activates when the sweeper is operating. The bubbler operates 24 hours per day, seven days per week when air temperatures are 20 °F (-6.7 °C) or less to prevent ice build-up on the screen.

4.1.16 Yellowhawk Weir and Fish Passage

A weir with wooden stoplogs is located at the second division works where Yellowhawk Creek starts. The weir is designed to create enough water depth for the Garrison Creek fish screen to pass sufficient water volume and work properly. There is a roughened channel (made of boulders) to create small steps for fish to ascend to transition between Yellowhawk Creek and the pooled water. The roughened channel keeps water velocity low enough for fish to navigate.

On the downstream side of the screen, a small slide gate regulates flow into Garrison Creek.

4.1.17 Project Office, Rooks Park and Related Facilities

The project office is near the First and Second Division Works and is used by Project personnel and also provides areas for storage of supplies and equipment. There are four other buildings nearby: a storage and maintenance/chemical storage building, shop, equipment shed, and pump house. All structures, equipment, and chemicals used or stored in this area meet all Federal and State regulations. The area also includes a parking area, public restroom, lawn area, bulletin board, and drinking fountain.

Rooks Park is an 18-acre day-use park located along Mill Creek near the north levee. The lawn area has a sprinkler irrigation system supplied by water from a well located in the maintenance building. The pond at Rooks Park is part of the original Mill Creek channel and is supplied by water from Mill Creek, which flows in a small channel through the park. The water entering the park from Mill Creek is screened to prohibit fish from entering the channel and pond. Flow is maintained through the pond year round to avoid stagnation and provide nuisance vegetation abatement. Flow returns to Mill Creek through a culvert with a flapper valve.

4.2 Project Maintenance and Future Construction

Appendix A, Chapter 2, in the 2018 BA includes a discussion of maintenance activities. Much of that discussion is copied or summarized below; for more detail refer to the appendix. The following sections summarize those project features, and also include certain improvements to the features that were listed in Appendix B of the 2018 BA. Figure 2 below provides an overview of the Project footprint.

Figure 2. Overview of the Project footprint (from figure A-1 in 2018 BA, Appendix A; numbers refer to sections in the BA)

4.2.1 Mill Creek Diversion Dam and Associated Structures

Mill Creek Diversion Dam and Forebay

Normal maintenance of the dam consists of visually inspecting the concrete for cracks and cleaning sediment deposits from inside the dam. The relief drain holes in the downstream face of the dam are also cleaned annually, which is done in the late summer because it is a relatively dry time of year. Mowing/grazing in the forebay area occurs during low water periods.

Some maintenance activities require lowering the Mill Creek Diversion Dam forebay (Table 1).

Lowering the forebay to an adequate level can only occur when Mill Creek flows are less than 100 cfs (as measured at the USGS gage near the Project office). To draw down the forebay for maintenance, the low-flow outlet radial gate is opened slowly over a 2- to 3-hour period to lower the water level below the intake canal headworks gates.

The Corps typically limits these annual maintenance activities to a single, 8-hour period in January or February, (forebay lowered to 1,252 feet), and a 5-day period in August or September during low flows (forebay lowered to 1,248 feet). Occasionally, there are other times that the forebay may need to be lowered for short periods of time, on an as needed basis. Table 1 lists the maintenance activities that require the forebay to be lowered, as well as frequency and the time of year these maintenance activities occur.

Table 1. Maintenance Requiring the Forebay to Be Lowered Maintenance Activity Frequency Timing

Forebay – sediment removal Every 5-10 years August/September Low-Flow Outlet Gate – gate maintenance

Every 6 months August/September

Low-Flow Outlet Gate – cable maintenance

Annually August/September

Low-Flow Outlet – remove debris blocking low-flow outlet

Periodically As needed

Fish Ladder Exit – install and maintain fish monitoring equipment

Annually Installation: January/February Removal: August/September

Intake Canal Headworks – gate maintenance

Annually August/September

Intake Canal Headworks – cycle gates

Biannually January/February August/September

Intake Canal Headworks – sediment removal

Annually August/September

Rotating Drum Fish Screens – sill maintenance

Annually June

(From Table A-2 in 2018 BA Appendix A)

The Corps will draw down the Mill Creek Diversion Dam forebay to 1,252 feet, when flows…