The seminal moment in restoring nearly 1,000 acres south of Sonoma into a tidal marsh takes place Oct. 25 when the levee is set to be breached, letting water flow in from San Pablo Bay for the first time in more than a century.

The initial filling of the tidal basin, which the public can witness for a short time on Sunday, is viewed as crucial to restoring Sears Point Ranch to its natural state prior to the land being diked in the mid-1800s and used for grazing.

The ranch spans 2,327 acres from San Pablo Bay across Highway 37 and up the hillside on the east side of Lakeville Highway near Sonoma Raceway. The property can be accessed via Reclamation Road, just south of the intersection of Highway 37 and Lakeville Highway.

Dozens of dignitaries and officials involved in the decade-long project will gather at the site Sunday for a private brunch and to witness the levee breach at noon.

The public is invited to view the return of the tides from 2 to 6 p.m. There is no cost to attend the event but the Sonoma Land Trust, the lead agency on the restoration effort, is asking people to register in advance online to get an idea of how many people will be attending, according to a spokeswoman. As of Thursday about 600 people had signed up.

For more information and to register visit www.sonomalandtrust.org/join_in/events.html.

Organizers expect the tidal basin will fill within 24 hours once crews using an excavator punch the 285-foot hole in the levee.

The area is not expected to re-open to the public until early 2016 when trail work and upgrades to a rail crossing are expected to be finished. The completed project will include a four-mile extension of the San Francisco Bay Trail, enabling hikers and bikers to travel from the Petaluma River to Tolay Creek.

Sears Point is the largest restoration and preservation project along the shores of San Francisco Bay since the purchase of 16,596 acres of Cargill Salt production facilities in the South Bay in 2002.

The land was used for cattle grazing and as the location of a hunting club, but over the years was the proposed site for developments ranging from an airport to a casino, which ultimately was built next to Rohnert Park.

Sonoma Land Trust raised $20 million to acquire the property in 2005 and another $18 million needed to complete the restoration work.

You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com. On Twitter @deadlinederek.