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.