I am not really to familiar with that area. I have been up the sac river but not that far. We really don't have a lot of barge traffic ( unlike FWL!). I have a hard time believing it was a freighter. I believe that any freighter has to hire a local captain to pilot the vessel before it enters the bay. I will keep my assumptions to myself but.....
I also find it strange he was anchored in the middle of the river, but as FWL commented might have drifted.
Here is a link to an explosion that happened a Bullfrog Marina a few days ago. This place I am familiar with, have passed by many times.
explosion