The park contains significant deposits of obsidian, which was historically quarried by indigenous peoples for tool-making.
Lake Ilsanjo was constructed in the 1950s by the park's former owners to provide water for cattle and irrigation.
The park's geology includes evidence of the Sonoma Volcanics, a sequence of rocks formed between 3 and 8 million years ago.
Trione-Annadel features one of the few remaining stands of coast redwood in the immediate interior Sonoma Valley region.
The park was named in part after the Trione family, who were instrumental in the land's conservation and transfer to state ownership.
Trione-Annadel State Park encompasses over 5,000 acres of rugged volcanic terrain in the Sonoma Mountains. The park is characterized by a diverse landscape featuring oak woodlands, grasslands, chaparral, and redwood forests. Lake Ilsanjo, a man-made reservoir, serves as a primary destination for hikers and anglers within the park. Over 35 miles of trails offer integrated routes for hikers, mountain bikers, and equestrians. The area is noted for its high geological variability, including outcrops of volcanic basalt and tuff. It serves as a critical habitat for mountain lions, bobcats, and diverse bird species. The park was transitioned from private ranch land to a state park in the 1970s. Visitors encounter distinct wildflower displays during the spring months. The site is currently fully operational and managed by California State Parks.
The shoreline of Lake Ilsanjo during the late afternoon provides reflections of the surrounding hills.
Download offline trail maps as cellular service is intermittent or non-existent throughout the interior trails.
Bring ample water, as there are no potable water sources available once you depart the main parking area.
Check the park's bulletin board at the Channel Drive entrance for current mountain lion sighting notices or trail closures.
Attempting to reach Lake Ilsanjo via the shortest trail without sufficient water, as the climb involves significant elevation gain in exposed areas.
Stay on marked trails to protect sensitive habitat and prevent erosion. Dogs are strictly prohibited on all trails; leashed dogs are only allowed in parking areas.
