 After re-entering Clackamas County, it passes the USGS gauge station at Milwaukie, from the mouth.Shortly thereafter, Johnson Creek empties into the Willamette River above its confluence with the Columbia River, which in turn flows about another to the Pacific Ocean at Astoria.The floodplain of Johnson Creek is a remnant of large glacial floods known as the Missoula Floods that deposited thick sediments between 18,000 and 13,000 years ago in the Columbia River Basin, including much of the Willamette Valley.