Condit Dam Removal Update #4

November 2011 View of #2 (upstream) coffer dam.

There’s been reference to “coffer dams” (or cofferdams) on this site, but what does the term mean?   A coffer dam is defined as a watertight enclosure from which water is pumped to expose the bottom of a body of water to permit construction of piers, dams, etc.

One of the many fascinations of the Condit Dam breaching was re-emergence of the original coffer dam used to re-route the river during construction 99 years ago (see Condit Dam History Part 5).  All of the wood submerged by the reservoir  seems to have been well preserved, including tree stumps, coffer dams, flumes and crib walls.   

This outcome has some real significance in terms of river restoration.  On the negative side, Coffer Dam #2 is now preventing the exit of some sediment from the upstream river canyon.  If left in place, the dam would also limit upstream passage of fish.  To alleviate the blockage, the coffer dam, and adjacent crib wall that originally directed water into the bell mouth of Tunnel #1, will be removed this winter.  The structures will soon transition into history. 

On the positive side, however, the preservation of tree stumps below the reservoir shores has provided the river restorationist with an accurate record of the species, density and size of the trees that grew along the canyon 99 years ago.  They also define the pre-project ground topography, which will be important in devising regrading plans.  This data has special importance, since there is very little photographic, map or written description of the canyon before 1912.

The photo below provides a glimpse of the untouched canyon above the dam.  As expected, the steep, dry, shallow-soil hillsides immediately above the river supported species such as Oregon white oak, dry adapted shrubs like mockorange and hazel, and scattered conifers.  Where hill slopes were less steep and supported deeper soils, the primary trees were probably a mixture of ponderosa pine and Douglas fir. 

(click on image(s) for larger view)

August 1912 View of #2 (upstream) coffer dam.
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6 thoughts on “Condit Dam Removal Update #4

  1. Please keep the photographs of the former reservoir coming. And, if possible, a specific date on each picture would be very useful. Many of us in the scientific community are looking at how the healing of the White Salmon River progresses as an analogue to other dam removals. I work on the Klamath River, and the reservoir restoration, and the fate of the river below the dam breach are of particular interest, given plans to remove the Klamath Dams in 2020. If you have any pictures from the head of the reservoir, it would be very interesting to see how the alluvial features of the river begin to re-express themselves.

    1. Hey Ross,
      Thanks for checking in. I’m actually working on a story for National Geographic about what’s happened since the breach. LOTS of changes–I was out at the river on Friday and it’s pretty incredible. Stay tuned–it’ll be worth the wait!

      Thanks,
      Andy

      1. Sounds great Andy. Thanks for the reply and can’t wait to see what you have in store for us next. Have a Merry Christmas!

  2. Curious how all you people who are so enthusiastic about the removal of this dam have nothing to say about how the manner in which is was removed devasted the lower White Salmon and the confluence with the Columbia River. The 4 million plus cubic yards of mud and debris that flooded downstream ruined one of the best sport fisheries in the northwest that hundreds of local fishermen and visitors had been using for decades. All the deep holes at Underwood now have 20 plus feet of mud and sand and the Indians boat ramp is landlocked and the bank fishermans bench is landlocked now. But it’s OK, it was all done for “the environment” and to make environmentalists feel all good about themselves. The government and the State of Washington should be sued and forced to remove all that mud and debris and restore the lower White Salmon at Underwood. Right now it’s just a wasteland.

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