Movable Bridges - Steamboat Slough Bridge

All the bridges at the southern end of the Sacramento River (north of the Rio Vista lift bridge) are heel trunnion double leaf bascule structures. They must have put a half dozen of these movable bridges between Sacramento and Rio Vista during the 1920s. Perhaps there was more ship traffic in those days. Riding my bike along the river today, all I see are a few speedboats. I wonder if a bridge operator is always on duty just in case?

Steamboat Slough Bridge doesn't actually cross the Sacramento River. It carries State Route 160 traffic over Steamboat Slough on the west side of the river. It's a 68.9 m long bridge that was built in 1924.

All of these bridges have height restrictions due to the huge, concrete counterweights (and low cross-beams). The bridges are almost identical with short spans to support the counterweights and long through truss movable spans. They just have different approach spans when a longer bridge is needed.

Movable Bridges - Steamboat Slough Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Movable Bridges - Walnut Grove Bridge

Continuing upstream along the Sacramento River we come to another double leaf bascule bridge. It is much like the previously viewed Isleton bridge, but without the tied arch eastern approach. The river has narrowed slightly where two branches come together in Walnut Grove. The bridge was built in 1952.

Movable Bridges - Walnut Grove Bridge by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.

Movable Bridges - Isleton Bridge (2)

Another view of the Isleton Bridge. It was built in 1923 and possibly widened (it currently has one lane in each direction) or maybe some other work was done in 1953.

I'm looking at the bridge log, which says it has nine spans (I count eight), its 190.2 meters long (why metric units?), and its rated GGGGG (not for permit vehicles).

The river doesn't look very deep. I often see dredging equipment working on the shipping channel, which must quickly fill with silt. The only collision protection appears to be a single timber pile on each side of the piers supporting the heel trunnions.

I guess it's remarkable that the bridge has survived for so long. No collisions, no flood damage, and no earthquakes. I don't see a seismic retrofit, but maybe short spans on pier walls aren't particularly vulnerable. In the Google Earth photo below we can see the Sacramento River emptying into the Delta at the bottom left and the Isleton Bridge at the upper right.

Movable Bridges - Isleton Bridge (2) by Mark Yashinsky is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 United States License.