Complex sentence: In the 21st century, the creek flows underground through a pipe that empties into the river Outfall 11, upstream of the Willamette's mouth on the Columbia River, which is about a quarter-mile downstream of the Broadway Bridge.
Split into simpler sentences: In the 21st century, the creek flows underground through a pipe that empties into the river at Outfall 11. The outfall is upstream of the Willamette's mouth on the Columbia River; that is, about a quarter-mile downstream of the Broadway Bridge.