Note: For many non-migratory species, occurrences are roughly equivalent to populations.
Estimated Number of Occurrences: > 300
Comments: Specimens recently identified as this species have been collected in parts of the Willamette and Columbia Rivers (Nedeau et al., 2005). Mock et al. (2004; 2005) found a lack of resolution in phylogenetic reconstructions of Anodonta populations in the Bonneville Basin, Utah, but there was a tendency for the Bonneville Basin Anodonta (tentatively A. californiensis) to cluster with A. oregonensis from the adjacent Lahontan Basin in Nevada. Mock et al. (2004) also cite collected specimens tentatively identified as Anodonta cf. oregonensis from the Burnt River, Baker Co., Oregon, and Ten Mile Creek, Elko Co., Nevada. If valid, this species is reduced to only a couple of localities in Utah in 3 counties in the north central part of the state (Oliver and Bosworth, 1999). Preliminary analysis (K. Mock, Utah State University, pers. comm.) indicates Utah Anodonta are distinct from Anodonta oregonensis of the Pacific northwest and should tentatively be assigned to Anodonta californiensis pending future taxonomic work. In Canada, this species, if valid, occurs across southern British Columbia including the Gulf Islands (Metcalfe-Smith and Cudmore-Vokey, 2004). In Oregon, several populations of Anodonta were recently confirmed in the Middle Fork John Day River and the lower main stem of the Umatilla River, but due to the taxonomic confusion surrounding the western Anodonta, identification to species level was not attempted (Brim Box et al., 2003; 2006), although preliminary evidence indicates the John Day River population includes A. californiensis/nuttalliana clade and the Umatilla River population include both A. oregonensis/kennerlyi and A. californiensis/nuttalliana clades in sympatry (K. Mock, Utah State University, pers. comm., 2007). Frest and Johannes (1995) list occurrences on the Okanogan River in Washington, Snake River in Washington and Idaho, and mountain lakes in the Washington cascades. Frest and Johannes (1999) list potential occurrences in the Umpqua River drainage in southwest Oregon. Chong et al. (2007) used specimens from near the Columbia/Willamette confluence (Bybee Lake and the Columbia Slough). Museum specimens (UMMZ) exist for California (South Fork Eel River in Mendocino Co., Mountain Lake in San Francisco Co.), Utah (Mill and Six Mile Creeks in Weber Co.; however Utah records are likely a different species of Anodonta), Oregon (Crump Lake and Cottonwood Creek in Lake Co., Silvies River in Harney Co., Klamath and Agency Lakes in Klamath Co., Lost River in Klamath Co., Upper Columbia River in Wasco Co., Willamette River, Oswego Lake in Clackamas Co.), Washington (Whatcom Lake in Whatcom Co., Spanaway Lake in Pierce Co., Snake River in Walla Walla Co., Lake Sammamish in King Co.), Nevada (Humboldt River, Big Springs Creek in Elko Co.), and British Columbia (Sumas River, Shuswap, Sumas, Glabnola, Chilliwack, Long, Okanagan, Small Lakes; and on Vancouver Island).