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.