One might call into question why this landlocked hill should be called and island. Historically, the name fit a bit better than it does today. Deer Island was a rocky high point in the middle of the Petaluma River Delta. The flat, marshy surroundings restricted access to an extent, much like that of an island.
Today access is far easier and thus it's far easier to get to the preserve's trails. The 154 acre preserve abuts the aptly named Deer Island Lane and includes a few parking spots at the trailhead. Looking at a map of the area it would appear that there's not much here in the way of trails. But in reality. several miles of trails await the visitor.
The Deer Island Loop Trail circumnavigates the island, providing views of the flat marshy areas around it. The De Borba Trail leads toward the high point of the island and should provide even better views. The vegetation is a mix of clusters of oaks and grassland. Don't be surprised to see a flock of turkeys crossing the trail or scavenging through the tall grass.