Boundary Dam Spillway No. 1 TDG Modifications

Project Summary

The historical Boundary Dam modifications project is located on the Pend Oreille River near Metaline Falls, Washington, north of Spokane. Boundary Dam consists of an arch dam, reservoir, and underground power plant and is the third of five dams on the Pend Oreille River.

The spillways on this scenic dam are cut into the rock and oriented in a way that allows full capacity water discharges from both sides.

After major safety considerations were taken surrounding modifications made to the spillways in 2016, it was decided that the TDG (total dissolved gas) was too high and affecting the fish downstream and the spillway surface should be returned to as near its original condition as possible.

IMCO was contracted by Seattle City Light to complete the removal of TDG modifications previously installed on the spillway. This included removing ramps and roughness elements embedded in the spillway concrete, decommissioning post tensioned anchors, removal of stainless-steel ramps, reinforced concrete infill to match the pre-TDG modification spillway surface, spill containment, and other environmental controls.

Boundary Dam now has a safer spillway design, less mist impacting the effectiveness of sensors, and less TDG environmental impacts downstream.



Metaline, Washington


City of Seattle/Seattle City Light

Contract Amount



July 2021 - September 2021

Project Highlights

Boundary Dam is the largest underground power generation dam in the nation and provides nearly 30% of the power generated by Seattle City Light.