|Condit Dam seconds after a blast has opened|
a gaping hole at the base on 10/26/11.
PacifiCorp via AP photo from here.
The Condit Dam is the second-tallest dam to be demolished in U.S. history, and it provided power for about 7,000 homes but the owner, PacificCorp elected to remove it rather than install expensive fish passage structures that would have been required for relicensing. The dam removal project will cost about $32 million when restoration of the valley is completed--and restoration will be needed as you'll see from the video below.
Prior to the blasting, work crews created an 18 foot wide, 13 foot tall tunnel in the base of the dam. A layer of silt about 50 feet high had accumulated in the lake, Northwestern Lake, behind the dam.
Here's a video of the actual event. Nothing happens for a minute (so you can keep reading this while you are waiting, but it's worth the wait!) Watch at the very base of the dam at about 1 minute 17 seconds into the film-- you can see a small atmospheric shock wave and steam cloud formed by the explosion that triggers a massively high-pressured jet. By breaching the dam at the bottom, sediment is removed from the base of the reservoir and yet the dam remains to be removed by heavy machinery at a later time. It may be my imagination but at about 1'27" it looks like branches in front of the camera start moving because of wind generated by the atmospheric shock and the violent flow. At about 2:15 there's a nice shot of the jet at the base of the dam with sediment coming out on the bottom and clear water on the top, a stratified flow.
(There are 3.3 miles of the white Salmon below the dam---No need to feel sorry for any luckless salmon that happened to be trying to return to spawn on this day....Fisheries biologists captured and relocated 679 tule chinook from below the dam to protect their spawning nests from the sediment removed from the reservoir. )
Removal of the dam will open 33 miles of habitat for steelhead after the restoration, and restore the White Salmon River for white-water rafting. Other dams in the process of removal and restoration are the 210-foot Glines Canyon Dam and the 108 foot Elwha Dam, both on the Olympic Peninsula.