Thompson Falls Fish Ladder – 48 Steps to Bull Trout Restoration on the Clark Fork River

The Thompson Falls Hydroelectric Project was constructed in the early 1900s, taking advantage of the natural falls along the Clark Fork River that gave the town of Thompson Falls, Montana its name.  The hydropower complex consists of two dams and two powerhouses, owned and operated by PPL Montana.  The dams at Thompson Falls have been a barrier to upstream fish migration since their construction.

The waters of the Clark Fork River are habitat to bull trout (a Federally-listed threatened species) and the westslope cutthroat trout (a Montana Species of Special Concern), as well as other native and non-native fishes. The Clark Fork River is a tributary of the Columbia River, upstream of the upstream-most range of anadromous salmon. It was not until the recent advent of radio telemetry - capable of tracking fish movements - that the impacts of blocking the migration of non-anadromous fish began to be appreciated.

In the early 2000’s PPL Montana and Montana Fish, Wildlife, and Parks biologists found that large numbers of fish of many species had their upstream migrations blocked by the dams.  An extensive radio telemetry study of migrating salmonids was conducted between 2003 and 2007.  The fish tracking revealed that fish were migrating up to, and holding at the main dam, which is the upstream-most dam at the complex. Fish that were transported and released above the dam were migrating as much as 100 miles to upstream spawning tributaries, highlighting the large geographic impact of the blocked fish migrations.

PPL Montana established an Interagency Technical Advisory Committee (TAC) to address fish passage issues at the Thompson Falls Project. Based on fish behavior studies and engineering review, the TAC came to a consensus decision that a full height fish ladder should be constructed on the right bank of the main dam to provide fish passage. GEI Consultants, Inc. designed a 48-pool fish ladder on the right abutment of the main dam with sampling facilities and attraction flows to restore upstream fish passage to this reach of the river.  Construction of the upstream fish ladder began in July 2009 by COP Construction Co. and was completed in 2010.   Full fish ladder operation will begin in March, 2011.  This paper discusses aspects of the ladder planning, design, construction, and operation.