The City of Petaluma says Shollenberger Park was originally a ranch the city bought in 1970 as a dredge-spoils site, a place to dump mud dredged from the Petaluma River. Today the 165-acre open space is both the largest and the most visited park in Petaluma.
The Petaluma Wetlands
Together with neighboring Alman Marsh and the Ellis Creek Water Recycling Facility, the area is commonly called the Petaluma Wetlands, the city says. The wetlands act as a natural floodplain for the tidal Petaluma River. Wikipedia says Alman Marsh covers 80 acres and Ellis Creek about 260 acres, and that Ellis Creek opened to the public in July 2009, bringing the accessible total to about 505 acres.
Wikipedia says the park is named for Richard Shollenberger, a park chief, and draws about 150,000 visitors a year.
Birds
Wikipedia says the last survey counted 231 bird species and that the Audubon Society ranks the park as an important birding site. Its species list includes grebes, herons and egrets, many kinds of ducks, raptors from bald eagles to white-tailed kites, and rails including the Ridgway's rail. The Petaluma Wetlands Alliance, a nonprofit, does habitat restoration and leads bird and wildlife tours.
Walking it
The city says the park is known for its 2-mile loop trail and a 1-mile cutoff, both popular for wildlife viewing. The entrance is at 1400 Cader Lane. For hills instead of marsh, try Helen Putnam Regional Park on the other side of town.