PPL Montana builds a one-of-a-kind fish ladder at the 94-MW Thompson Falls Project for the passage of migratory bull trout and other resident fishes in Montana.
By Jon Jourdonnais, Brent Mabbott, Chad Masching and Ginger Gillin
Clark Fork River is the largest river in Montana, with an average annual discharge of approximately 20,000 cfs. The river’s headwaters start at the Continental Divide, just east of Butte, Mont., and flows through western Montana entering Lake Pend Oreille, west of the Idaho/Montana state boundary.
Meriwether Lewis explored this river during the Lewis and Clark expedition’s 1806 return from the Pacific, although the river is named after his partner, William Clark. The Metaline Falls, downstream of Lake Pend Oreille, prevented the establishment of anadromous fishes. Thus the Clark Fork River was home to resident trout and other fish species capable of completing their life cycle without migrating to the sea.
|The Thompson Falls fish ladder was opened for fish passage on March 16, 2011, giving two rare species of trout safe passage into their native spawning waters. (Photo courtesy of PPL Montana)|
The Clark Fork River has three hydroelectric projects in its lower reaches. The 94-MW Thompson Falls Hydroelectric Project is the oldest and smallest of the three, being commissioned in 1915. It is owned and operated by PPL Montana, LLC. None of the three hydro projects were built with passage facilities for upstream migrating fish, due most likely to a lack of knowledge of the migratory nature of river fishes at the time of construction.
In recent years, with innovation of small radio transmitters capable of tracking the movement of fishes, biologist’s understanding of the migratory habits of resident fish has been enhanced. It is now understood that resident fish migrate long distances within river systems to reach their natal streams for spawning. Blocking those migrations has had a deleterious effect on the reproduction of these fishes. In 1999, the bull trout were listed as a threatened species under the Endangered Species Act. The lack of passage through the three dams on the Clark Fork River was one reason cited for the decline in the abundance of this fish.
Need for fish passage
Bull trout are present in the Clark Fork River upstream and downstream of the Thompson Falls Project. A draft Biological Evaluation was prepared by PPL Montana and submitted to the United States Fish and Wildlife Service and the Federal Energy Regulatory Commission in 2003. The Biological Evaluation assessed impacts the dam and powerhouse may have on bull trout and made recommendations for conservation measures to reduce those impacts.
Section 7 of the Endangered Species Act requires federal agencies to ensure on-going project actions are not likely to jeopardize the continued existence of federally listed threatened or endangered species, or to destroy or adversely modify designated critical habitat.
The Thompson Falls Biological Evaluation concluded that the project was likely to adversely affect bull trout due to the lack of upstream adult fish passage, potential for delay or mortality during downstream passage and potential impacts from increases in total dissolved gas during high spill periods. PPL Montana established an interagency Technical Advisory Committee to assist with development of conservation measures for bull trout.
Committee members include PPL Montana, Montana Fish, Wildlife and Parks; the USFWS; the Confederated Salish and Kootenai Tribes; Montana Department of Environmental Quality; and others.
Initial research at the project included collecting fish at the Main Dam spillway to assess species, numbers and timing of fish attempting to pass the Project. Bull and westslope cutthroat trout, a Montana State-listed Species of Special Concern, were radio-tagged, trucked above the dam, and released.
Signals from radio tags were tracked so that biologists could assess trout movement and determine the location of spawning areas. Data collected during this research indicated that large numbers of a wide range of species were attempting to migrate past the Thompson Falls project. In addition, bull trout and westslope cutthroat trout released upstream of the Project were found to spawn in tributary streams over 30 miles and 100 miles, respectively, upstream of the project.
|The Thompson Falls project includes two powerhouses, separated by an island, and two spillways, also separated by an island. (Photo courtesy of PPL Montana)|
Fish ladder site selection
Decisions about the location of a potential fishway were complicated by the complex nature of the Thompson Falls Project configuration.
There are two powerhouses, separated by an island, and two spillways, also separated by an island.
Would fish be more inclined to approach the powerhouses, which during some periods had the greatest attraction flow? Or would they migrate to the most-upstream area of the tailrace, below the Main Dam Spillway?
To answer these questions, a fish behavior study was designed and implemented to determine the best location for a permanent fish passage structure. Trout species were collected in the tailrace, radio tagged, and released. Their movements as they approached both dam spillways were tracked. Experiments with spillway opening plans were conducted to determine if fish behavior could be influenced by modifying hydraulic conditions in the tailrace.
The conclusion of these studies was that fish approached the Main Dam spillway when migrating upstream, and furthermore could be attracted to the Main Dam right bank by opening panels on the spillway in a specified order.
Fish ladder design
With collaborative input from the TAC, GEI Consultants, Inc. was contracted by PPL Montana to design a 48-pool reinforced concrete fish ladder structure to allow unhindered passage of fish through the 48 vertical feet between the tailwater and normal forebay pools.
Each 5-foot-wide pool in the ladder has a nominal depth of about 4 feet. A 1-foot drop between pools is induced at weir plates installed at the downstream end of each pool. To give the fish ladder additional flexibility, the aluminum plates were fabricated with an orifice at the bottom, a weir notch at the top, and a sliding plate between the two that allow biologists to operate the ladder in either weir or orifice mode. The orifices and weir notches were both sized for a 6 cfs pool-to-pool flow. The top three pools in the ladder have permanent orifice plates, which maintain ladder inflow near 6 cfs with varying tailwater elevations.
Three gates downstream of the intake structure allow diversion of AWS flows to a stilling basin, the sampling facilities, or the upstream side of the ladder entrance (Pool 1). Supplemental attraction flows of 20 to 60 cfs can be added into Pool 1 (ladder entrance) from the AWS stilling basin. A 20 cfs high velocity attraction jet can also be discharged in front of the ladder entrance. These two attraction flow sources, coupled with selective opening of flashboards on the Main Dam are used to attract fish to the right abutment of the Main dam and into the fish ladder entrance gate.
At the top of the fish ladder, a sampling loop system and handling facilities were designed to allow biologists to process all fish passing through the ladder. At Pool 45, fish can be routed into a holding pool, where they are guided to a vertical lock. Water is pumped into the lock to lift the fish approximately 15 feet to a sampling platform.
PIT tag readers were installed in the lower ladder and at the holding pool entrance so biologists can estimate fish movement rate and percentage of fish that fall back down the ladder prior to being handled.
|This 800 foot long, 27 foot high steel and timber work bridge provided access for heavy equipment and acted as a crane platform to the Thompson Falls fish ladder. (Photo courtesy of PPL Montana)|
The fish ladder construction contract was awarded to COP Construction, LLC, based in Billings, MT. Morrison-Maierle Inc. provided construction administration services. Initial construction activities consisted of rock excavation near the right abutment of the Main Dam and tailwater access bridge construction.
To fit the switchback layout of the upper fish ladder pools, the contractor was required to carve and bench the argillite bedrock rock face using excavators with hydraulic rams. While excavation proceeded along the right abutment, COP crews erected a work bridge along the downstream concrete apron of the main dam. This 800 foot long, 27 foot high steel and timber work bridge provided access for heavy equipment and acted as a crane platform to the fish ladder. Construction access to the ladder from the right abutment of the dam was limited to small vehicles due to a load limited bridge.
Upon completion of the work bridge, soil, cobbles and debris from the footprint of the ladder were hauled off site. The native argillite excavated from the rock face of the abutment was permitted by U.S. Army Corps of Engineers 404 permit to be placed in the tailrace. Approximately 650 cubic yards of mass concrete were placed in the excavation to bring the fish ladder foundation to grade and over 2,850 lineal feet of rock anchors were installed to add stability to the excavated rock slope.
The reinforced concrete fish ladder was constructed from the bottom up over the course of about 16 months.
A 42-inch diameter tunnel was cored through a 10 foot wide concrete section of the Main Dam Spillway at the right abutment to allow the ladder to operate under gravity flow and provide the ability for fish to pass through the ladder without being handled. A reinforced concrete exit pool structure was constructed behind a small cofferdam. A 23,500 pound auxiliary water intake structure and traveling screen were installed with divers and anchored at the upstream end of the existing log sluice.
The ladder and appurtenant facilities were substantially completed in September 2010 and was passing water for the Sept. 8, 2010, inauguration.
First year fish passage results
The fish ladder was opened for fish passage on March 16, 2011. The first fish, a rainbow trout, was collected at the sampling facilities at the top of the ladder on March 21, 2011.
The first bull trout was collected on April 13, 2011. Unusually cold weather kept water temperature below normal all spring, likely reducing the numbers of fish migrating in March and April. Western Montana had an extremely high snowpack in 2011, and spring runoff was one of the largest on record in terms of total volume of water. High flows forced the closing of the ladder on May 24, 2011. As of that date, a total of 114 fish of nine different species had successfully ascended the new ladder.
The 2011 spring and early summer runoff peaked at approximately 110,000 cfs, which is about 43 percent higher than the mean peak flow. Although the high tailrace waters overtopped the exterior walls of the ladder and allowed debris and sediment to accumulate within the lower pools and stilling basin, significant damage to the ladder was avoided.
On June 9, 2011, PPL Montana was required to trip stanchions in several bays on the Main Dam to pass excess water and debris from the swollen Clark Fork River. Once flood waters receded, the forebay water elevation was reduced by 16 feet to make stanchion repairs. PPL Montana reconstructed these spillway stanchions and returned the fish ladder to operation August 22, 2011. In the first week after the ladder was re-opened, over 1100 fish of seven different species ascended the ladder and were passed upstream.
Jon Jourdonnais is the Manager of Hydro Licensing and Compliance for PPL Montana. Brent Mabbott is a Principal Compliance Professional for PPL Montana. Chad Masching is a Civil Engineer for GEI Consultants, Inc. Ginger Gillin is a Principal Environmental Scientist for GEI Consultants, Inc.