I found a bike club website with directions for a ride to the Grizzly Island Wildlife Area in the Delta. It noted the route could be extended to the far side of the island for riders willing to grind gravel. I combed through the California Department of Fish and Wildlife website and the Grizzly Island website to see what they have to say about bikes, but I couldn’t find anything one way or the other. I emailed the relevant CDFW office and asked if bikes are allowed, but received no response.
Even if it turned out they don’t allow bikes on trails, certainly bikes would be okay on the main (gravel!) road across the island, which looked interesting.
PS: At my “Bill Bikes” website I recently wrote some code to add a new feature. For each day of the year, the code dredges up my ride(s) on the same date in previous year(s). Today, for example, “This Day in Biking History” contains a link to my ride through the Estero on 16 July 2018. Check it out.