CA DFG did an assessment of potential mussel success based on calcium levels at lakes around the state.

http://www.nrm.dfg.ca.gov/FileHandler.ashx?DocumentID=3873

From everything I can tell the mussels would have no problem living in the fresh water portions of the Delta. They don't live in saltwater.

The Delta has what they call a hydrological barrier during the summer months. Basically a narrow section of the Delta where the water goes from fresh to salt. This is maintained through releases from the lakes upstream. So the dividing line of mussel infestation would probably be right along that hydrological barrier (a few miles of distance down past Sherman).