East Side Big Pipe
The East Side Big Pipe project , begun in 2006 and finished in 2011 , was the largest of a 20-year series of projects designed to nearly eliminate combined sewer overflows ( CSO ) s into the Willamette River and the Columbia Slough . 
<triplet> Willamette River <subj> Columbia Slough <obj> tributary <triplet> Columbia Slough <subj> Willamette River <obj> mouth of the watercourse