I found some photos I took of the East Channel bridge being removed. I think these were taken about May 1982. I'm not sure when this version was built. History Link has some information
here about the first bridge in that location opening in 1923 and refers to a newer bridge being completed by 1940 when they removed the 1923 one. So I think this bridge was built sometime in the 30's.
Anyone remember how long they had to close down this portion of I-90 between Belleve and Mercer Island?
Here's a great post by Kurt Clark on the Lacey V. Murrow and Homer M. Hadley floating bridges which continues I-90 from the west side of Mercer Island over to Seattle.