The Small Dam That Helps Hold the Lake: The South Channel Story at Post Falls

At Post Falls, the Spokane River does not go over a single falls. About nine miles downstream from the outlet of Coeur d'Alene Lake, the river divides into three channels around rocky islands, drops over a natural falls in each channel, and then rejoins below the gorge. Most visitors notice the brick powerhouse in the middle channel or the broad spillway of the north channel dam, viewed from Falls Park. The south channel dam is smaller, quieter, and easy to overlook, yet the whole system depends on it.

Washington Water Power, the company now known as Avista, completed the south channel dam in 1906 as one of three dams built across the three channels in only two years. It is a concrete gravity dam, 25 feet high and 127 feet long, fitted with six steel vertical sluice gates. Unlike the middle channel, it has no turbines and generates no electricity by itself. Its work is control. Together with the north channel dam, it passes water safely in periods of high flow, helps maintain water levels in the forebay in times of low flow, and helps Avista control the level of Coeur d'Alene Lake and parts of the Coeur d'Alene, St. Joe and St. Maries rivers during the summer recreation season.

For more than a century, that control was exercised the hard way. The south channel gates were operated manually, and by 2014 the structure was 108 years old, still carrying its original concrete facing and gate frames. Avista had already upgraded the middle and north channel dams in 2012. The smallest dam was next, and it needed more than a coat of paint.

In March 2014, Avista selected Spokane-based Max J. Kuney Co. for a 13.5 million dollar upgrade and restoration of the south channel dam. The plan called for removing the original concrete facing and gate frames and adding new concrete, spillway gates and hoists. Remote control equipment would replace the manual operation. To do the work in a live river, the contractor built a temporary cofferdam just upstream, both to dewater the work area and to provide a platform for a construction crane. Denver-based AECOM served as civil engineer, while Avista designed some of the electrical controls in-house. Most of the work was to be done with the river and the lake at full summer pool, with minimal or no impact on recreation.

Avista announced the completed work on May 5, 2016, saying the 1906 dam had been completely upgraded and refurbished over the previous two years. It emerged with six new spill gates, frames and hoists, a new spillway control system, new electrical distribution equipment and new concrete facing. In Avista's words, the historic structure would continue its role in providing summer recreational opportunities and spring flood control on Coeur d'Alene Lake for another 100 years. From Black Bay, downstream and around the bend, that is easy to forget on a calm summer day, when the lake looks natural and inevitable. It is not. Part of the reason it holds its summer line is a small dam most people never see, rebuilt gate by gate so it can keep doing its quiet work.