Trout Lake Marsh is located in the northwest corner of Klickitat County, just north of the town of Trout Lake. It has open fresh water, marsh, and riparian wetlands, with aspen stands and Oregon white oak woodlands on the higher ground. Most of the site is managed as a Natural Area Preserve by the Washington Department of Natural Resources. The site receives high recreational use. It has been identified by The Nature Conservancy as a high-priority site for enhanced protection.
The extensive marshes and adjacent cottonwood forests comprise one of the largest intact wetland ecosystems in south central Washington, and support an assemblage of birds associated with these habitats. More than 150 bird species are known to use this site, including an exceptional diversity of breeding neotropical migrants. At least 50 species, including 10 species of warblers, are known to nest here. Listed or candidate species include Bald Eagle, Sandhill Crane, Vaux's Swift, White-headed Woodpecker, and Loggerhead Shrike. WatchList species include Long-billed Curlew, Band-tailed Pigeon, Short-eared Owl, Rufous Hummingbird, and Hermit Warbler. Washington's westernmost breeding records of Veery and Gray Catbird were obtained at this site. Trout Lake Marsh is one of only three sites in the state where the Oregon spotted frog is known to live. High-quality communities of pale blue-eyed grass, a state-threatened species, and the state-sensitive Pulsifer's monkey-flower occur on the site.