Freshwater Mollusk Water Quality Tolerances and Observations About the Effects of Dams in the Middle Snake River Sub-Basin, Southern Idaho

Authors: Peter Bowler, Terrence Frest

Year: 2018

Abstract

Relative water quality sensitivities of 59 native and introduced freshwater mollusk species from thirteen families within the Middle Snake River sub-basin, southern Idaho, are presented. When ranked on a scale of ascending tolerance from 0 - 10, within the native fauna 11 taxa are very sensitive to water quality and fall within the 0 - 2 range, 13 species lie within the 2 - 5 range, and a group of 16 species can tolerate water quality within the 6 - 10 range. Three exotic species are very pollution tolerant, as is another group of six non-native taxa occurring primarily in thermal plumes from warmwater hatcheries. The most sensitive species have declined significantly within the mainstem river and many now are abundant only in tributary springs. Several are locally extirpated and are known only from shells, and several are federally listed as Threatened or Endangered taxa. Other rare endemics such as Stagnicola hinkleyi (F.C. Baker, 1906) are near extinction. Even within the second tier of species, there has been continuing decline. The Idaho Department of Fish and Game studies in the late 1940s and early 1950s suggest an historic fauna dominated by Fluminicola spp. and other sensitive taxa, which was dramatically reduced by severe diurnal instream flow fluctuations from the operation of hydroelectric dams. Continued, though less severe, “load-following” dam operations and chronic subsequent poor water quality shifted the fauna to more pollution tolerant, ubiquitous taxa. The New Zealand mudsnail, Potamopyrgus antipodarum (Gray, 1843), is currently the dominant macroinvertebrate at most sites. Water quality tolerances are reported for 10 other native taxa in tributaries or elsewhere in the Snake River system.