Most of the range and population decline has occurred in Oklahoma and Kansas. The discovery of relic shells throughout Oklahoma rivers suggests that the species was once widespread in the state. Within Oklahoma, the Neosho mucket is however now restricted to a 55-mile stretch of the Illinois River between Lake Tenkiller and Lake Francis. Nevertheless, it has been recognized as a dominant unionid species inhabiting this remaining section of the Illinois River in Oklahoma, and mussel species richness in general may be declining in this state. Meanwhile, in the Arkansas portion of the Illinois River, there was a recorded 53% decline in the number of designated survey sites inhabited by the Neosho between 1998 and 2008. Further, the majority of subpopulations in Arkansas appears unstable and probably faces imminent extirpation.