Due to taxonomic uncertainty, it is difficult to determine the number of occurrences for this species (probably more than twenty but less than 100). Beetle (1989) lists the Bear River in Uinta Co., Wyoming. In Utah, occurrences (between two and six) are known from Utah, Millard, Rich, Tooele, and Box Elder Cos. (Oliver and Bosworth 1999). These sites were resurveyed recently and extant populations (tentatively
Anodonta californiensis) were found in Bear River, Redden Spring, Pruess Lake, Piute Reservoir, Otter Creek Reservoir, and Burriston Ponds, all in Utah (Mock
et al. 2004). Mock
et al. (2004) also cite specimens tentatively identified as
A. californiensis from the Black River, Apache, Arizona. Frest and Johannes (1995) report that the range has been reduced and extant populations are found in: the Middle Snake River in Idaho; the Fall and Pit Rivers in Shasta Co., California; the Okanogan river in Chelan Co., Washington; and Roosevelt and Curlew Lakes in Ferry Co., Washington. No living specimens were found in the Willamette and lower Columbia rivers in searches by Frest and Johannes from 1988-1990. Frest and Johannes (2000) list it as common locally in the Snake River and major tributaries. Taylor (1981) reports most California populations as eradicated and the species is probably extinct in most of the Central Valley. It was once distributed throughout six major drainages in Arizona (including Lake Mead, Grand Canyon, Lower Colorado-Marble Canyon, Lower Lake Powell), but today is only in portions of the Black River drainage and Little Colorado River (Nedeau
et al. 2005). In Utah, the only recent records are widely spaced, in Big Creek and Reddin Spring Pond, but it may still be extant in the Raft River and portions of the Bear River drainage (Clarke and Hovingh 1993). Hovingh (2004) found it widely distributed in the Humboldt River drainage (Lahontan Basin) in northern Nevada, the Bonneville Basin in Utah, Nevada, and Wyoming, and the Malheur and Warner Basins in Oregon. Other Nevada occurrences are in the Great Salt Lake, North Fork Humboldt, Truckee, and Carson Desert basins (NV NHP pers. comm. 2007). Mock
et al. (2004), in an analysis of genetic diversity, found the Bonneville Basin (Utah) population cluster with
Anodonta oregonensis from the adjacent Lahontan Basin (surveyed in Elko, Nevada) and the Middle Snake/Powder basin (Baker Co., Oregon). Mock et al. (2004) further differentiated Glenn and Solano Co., California, specimens as
Anodonta wahlamatensis, thereby limiting
Anodonta californiensis populations to Utah (see above) and Arizona (Black River in Apache). In Oregon, several populations were recently found in the Middle Fork John Day River and the lower main stem Umatilla River, but due to taxonomic confusion, identification beyond genus was not possible (Brim Box
et al. 2006); however, preliminary evidence indicates the John Day River population includes the
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). Chong
et al. (2008) utilized specimens from the East Fork Black River, Arizona, in their phylogenetic study. Recent Washington records are mainly from the Columbia and Okanogan Rivers and some ponds adjacent to the Columbia River (Nedeau
et al. 2005). In Arizona, Bequaert and Miller (1973) list the Lower Colorado, San Pedro-Wilcox, Chevelon, and Little Colorado drainages as historical with the only recent occurrence in the Black River drainage. Historically in Arizona, it was found in most drainages including the Black, Salt, Santa Cruz, Verde, Gila and Colorado Rivers, but today it is only found in the upper Black River in the the Apache-Sitgreaves National Forest to at least the White Mountain Apache Reservation (AZ NHP pers. comm. 2007). A population may be extant on Chevelon Creek according to Landye (1981).
In a survey of streams in California, approximately 8,000 individuals were found in the upper reaches of the Eel River (none in Ten Mile, Elder, or Fox Creeks), restricted to the lower 2 km of the upper portion of the river (Cuffey 2002).
Frest and Johannes (1995) report that the species is declining in terms of area occupied and the number of sites and individuals, and local declines have been observed throughout the range (Mock et al. 2010). It is generally widely distributed, though scarce; it is likely extirpated from the Colorado River basin in Arizona and Death Valley Basin, Los Angeles Basin, and Central Valley in Caifornia (Hovingh 2004). A recent survey of 115 sites in the Plumas, Tahoe, and Eldorado National Forests plus Lake Tahoe Basin management unit found no Anodonta specimens (in 70+ streams) except a few whole shells at 15 m depth in Donner Lake despite historical occurrences there (Howard 2008). The species is declining (possibly extirpated) in Utah with historic populations only in the Raft River (Box Elder Co.), Utah Lake (Utah Co.), and Bear Lake (Rich Co.) (Oliver and Bosworth, 1999). It is nearly extirpated from Arizona and has disappeared from the Sacramento River system. In Canada, this species occurs in British Columbia where it is declining (Metcalfe-Smith and Cudmore-Vokey 2004). Regardless of the taxonomic outcome of analysis of Anodonta molecular phylogeny, it is widely recognized that Anodonta in the western U.S. are in decline (Mock et al. 2004).
