Boundary Dam Spillway No. 1 TDG Modifications

Project Summary

Boundary Dam 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. The spillways on this scenic dam are cut into rock and oriented to allow full capacity water discharges from both sides.

After extensive consideration surrounding modifications made to the spillways in 2016, it was determined that the total dissolved gas (TDG) was too high, this was affecting the fish downstream. It was determeined that 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, and removing stainless-steel ramps. Crews also installed reinforced concrete infill to match the pre-TDG modification spillway surface and placed spill containment, and other environmental controls.

This project provided a safer spillway design at Boundary Dam, with less mist impacting the effectiveness of sensors, and the project decreased environmental impacts downstream due to TDG.



Metaline Falls, Washington


Seattle City Light

Contract Amount



July 2021 - September 2021

Project Highlights

This project won the ENR Northwest Best Project award, and then went on to win the national competition, the ENR Best of the Best Project in the small project category!