Dissolved calcium concentration [Ca 2+ ] is thought to be a major factor limiting the establishment and thus the spread of invasive bivalves such as zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels. We measured [Ca 2+ ] in 168 water samples collected along $100 river-km of the lower Columbia River, USA, between June 2018 and March 2020. We found [Ca 2+ ] to range from 13 to 18 mg L À1 during summer/fall and 5 to 22 mg L À1 during the winter/spring. Previous research indicates that [Ca 2+ ] < 12 mg L À1 are likely to limit the establishment and spread of invasive bivalves. Thus, our results indicate that there is sufficient Ca 2+ in most locations in the lower Columbia River to support the establishment of invasive dreissenid mussels, which could join the already widespread and abundant Asian clam (Corbicula fluminea) as the newest invader to an already heavily invaded Columbia River ecosystem. These new data provide important measurements from a heretofore undersampled region of the Columbia River and have important implications for the spread of invasive bivalves and, by extension, the conservation and management of native species and ecosystems.

The Asian clam, Corbicula fluminea, is an invasive freshwater bivalve that has established populations across the globe and is known to have deleterious effects on natural and human systems. Yet, despite being present in the Columbia River (CR) for nearly a century, little is known about this invader's basic biology and ecology in this large river system. Thus, we undertook a field study to assess its (i) broadscale distribution and abundance, and (ii) associations with habitat characteristics in the lower CR. During 2019-2020, C. fluminea were collected from 27 shore-based stations spanning 481 river kilometers of the lower CR, along with several habitat characteristics (bank slope, temperature, dissolved oxygen, pH, salinity, conductivity, chlorophyll-a concentration, and sediment composition and % organic matter [OM]). C. fluminea abundance ranged from 0 to 430 ind. m −2 . Most sites with abundances >100 ind. m −2 were located downstream of Bonneville Dam, whereas most sites with abundances <100 ind. m −2 were located upstream. Generalized linear models predicting the abundance of C. fluminea indicated significantly positive correlations with water temperature and % sand, and negative correlations with bank slope and sedimentary % OM. We also reviewed the global literature on abundance and habitat associations of C. fluminea and compared this with our own results. Our investigation represents the greatest spatial extent at which C. fluminea has been studied in the CR and our results provide a better understanding of the basic biology and ecology of this global invader, as well as provide natural resource managers with information on habitat conditions favorable for this invasive bivalve within temperate river ecosystems.