Baseline information on the current status of rare mussel populations in a given water body is necessary to better understand the effects of actions to protect or enhance populations. We conducted river-wide surveys in French Creek, known for its mussel abundance, to quantify population-level indicators that could be used for measuring viability of Epioblasma torulosa rangiana (Northern Riffleshell), a critically imperiled freshwater mussel, in recovering or reintroduced populations. We estimated multiple attributes of Northern Riffleshell populations, including longitudinal distribution, densities and abundances, sex-specific age structure, and mortality rates. Northern Riffleshell has been documented in French Creek since at least the early 1900s and is distributed unevenly throughout the creek (12 of 32 sites in a bimodal pattern), with no animals found in the upper third of the creek. At sites containing Northern Riffleshell, site-specific densities ranged from 0.009–6.668 m². Maximum age of Northern Riffleshell ranged from 7–11 years at four sites with evidence of sustained recruitment (i.e., uneven age structure). Proportions of individuals in each age class were similar at each site, even though total numbers of animals differed by up to two orders of magnitude. Significantly more males than females were found in early ages (1–3), but no significant differences were found in older age classes. There were no significant differences in mortality rates (both sexes combined) at all four sites. However, mortality rates differed significantly between the sexes at older ages (ages 6–10) at the one site with enough individuals for a comparison, suggesting greater reproductive costs or selective predation for females. The population attributes of Northern Riffleshell from French Creek are important benchmarks for setting restoration goals and measuring success in other systems that share a similar biogeography to French Creek, but whose fauna has been depleted (e.g., many Ohio River tributaries).