The last to be built, and arguably grandest, of the river bridges, the Sixth Street Viaduct was made eligible to be included on the National Register of Historic Places in 1986. Nearly a mile in length, connecting downtown LA to Boyle Heights, it has been included in numerous films, music videos, and tv shows, making it one of the most readily recognizable sites along the river. Unfortunately, when the bridge was built in 1932, concrete was used that has since been found to have a high alkali content, causing a chemical reaction within the concrete that has left cracking throughout the structure, compromising its strength. Today, it's predicted that the bridge has a 70% chance of collapsing during the next major earthquake, expected sometime in the next 50 years.
Above is a video clip of the impressive Sixth Street Bridge.