File:Cutting away the cofferdam at the Montlake Cut between Lake Washington and Lake Union (SEATTLE 1216).jpg

English:

Possibly not before 1916

On verso of image: Letting Lake Union into Lake Washington Canal - Dec. 1913

PH Coll 1099.36

• Subjects (LCSH): Montlake Cut (Seattle, Wash.); Cofferdams--Washington (State)--Seattle; Union, Lake (Wash.); Lake Washington Ship Canal (Seattle, Wash.); Montlake (Seattle, Wash.); Lakes--Washington (State)--Seattle; Canals--Washington (State)--Seattle

---

The above is from University of Washington Libraries.

The December 1913 date is almost certainly the right one. From the image, it's pretty easy to see that this is the west end of the Montlake Cut. David B. Williams, who presumably should know as well as anyone, writes, "Recognizing the need for a bigger canal, in June 1912 the Corps of Engineers hired Stillwell Brothers Construction Company to excavate a larger cut, 2,200 feet long, 100 feet wide at the bottom, and 36 feet deep. Stillwell finished in late 1913 and on December 31, 1913, blasted out the cofferdam (or temporary dam) on the west end of the canal. Water gushed into the cut and soon rose to the level of Lake Union, 20 feet above sea level but 9 feet below Lake Washington. Unfortunately, the south side of the cut was slumping, an issue that had plagued the project throughout construction but that engineers hoped would stop when water entered the cut. Stillwell built another cofferdam, drained the canal, and poured concrete on the slide area. Workers blasted that cofferdam in November 1914. But slumping continued, leading to another cofferdam in March 1916 and additional concrete work." (https://www.historylink.org/File/1444)