Eric Richardson [Flickr]

Marcello Vavala gives the history of the 7th Street Viaduct, built on top of an older bridge.

DOWNTOWN LOS ANGELES — Sunday's Bridge Mix event offered the public a chance to get up close to four of the historic bridges crossing the L.A. River as it passes through Downtown. An article in Monday's LA Times said that approximately 400 people turned up for the event.

In fact, the day should more correctly have been given the much less catchy "Viaduct Mix" moniker, as Conservancy docents were quick to distinguish that a bridge crosses only water. Since the bridges along the river cross railroad tracks and roads as well, they are in fact not bridges.

In our preview of the event, the Conservancy's Director of Education, Trudi Sandmeier, teased us that the 7th street span was an early example of adaptive reuse. The current viaduct was in fact built on top of an earlier bridge that crossed only the river.

As traffic increased and a streetcar line was added, the city needed a route that cleared the railroad tracks on the river's banks. The chosen solution involved building up from the existing bridge, leaving the odd double-decker look seen today.