Johnson Creek begins its 26-mile journey from pristine headwaters in the Cascade foothills near Boring, but as it snakes through Gresham, Portland and Milwaukie, it picks up a contaminated payload before spilling into the Willamette River. At an oxbow three miles from the mouth of Johnson Creek, it flows 400 feet from side-by-side Precision Castparts Corp. manufacturing plants along Johnson Creek Boulevard, which Portland regulators found have repeatedly discharged toxins into a storm pipe draining into the creek.