In the early part of the 1900's, the Great Northern Railway had a problem: Their coal-burning steam engines belched so much smoke and noxious gasses that the crew would nearly suffocate each time the train passed through the 3-mile tunnel at the top of this route. Passengers weren't too keen on this, either. The railroad experimented with many plans, including the use of some locomotives with smoke stacks running the length of the engine (to release the smoke behind the locomotive -- I assume they tried this on freight trains only, but don't know that for sure). However, none of these plans worked. Finally, in 1907 they began work on Tumwater Dam, in the hope that they could build a hydroelectric plant and keep some electric locomotives stationed here to pull any trains through the tunnel.
The dam and hydroelectric plant were completed in 1909. At that time this was the largest hydroelectric project west of Niagara Falls.
Apparently this plan worked, and the electric trolley system was used until 1929, at which time the new eight-mile tunnel (which is still used today) was completed. I'm not sure whether the trolley's were used in the new tunnel or if ventilation was better and locomotives "clean" enough by then to be able to pass through the new tunnel without serious problems.
Apparently electricity was produced here until 1956.
The dam has a length of 400 feet and stands 23 feet high.
For more (AFTER you watch this short video), take a look at the following websites:
From AUGUST, 2011: CHELAN AND BACK TOWARD SEATTLE