Examination of historical museum specimens coupled with modern samples can document faunal change over time. Samples from Big Lake in the Tippecanoe River watershed in northeastern Indiana were collected by Herman Wright in 1926–1927. Big Lake is one of the highest lakes in the watershed, so could be expected to be relatively pristine with little faunal change. From Big Lake samples, we identified 16 mollusk taxa (14,026 individuals) from 1926–1927 and 13 taxa (3,438 individuals) from 2020–2021, totaling 17 taxa in both time periods; relative abundances of taxa in the two time periods differed significantly (chi square, p < 0.001). Shannon diversity was smaller in the 1926–1927 sample (H' = 1.55) and slightly larger in the modern sample (H' = 1.80) despite more species found in the earlier time period, perhaps because species in the modern sample had greater evenness. Four species were relatively more abundant in samples from 2020–2021 than 1926–1927, Armiger crista (Linnaeus, 1758), Gyraulus deflectus (Say, 1824), Physella Haldeman, 1842, and Promenetus exacuous (Say, 1821), while two species were relatively less abundant in samples from 2020–2021 than 1926–1927, Galba humilis (Say, 1822) and Valvata bicarinata I. Lea, 1841 (p < 0.001 for all six comparisons). The invasive clam, Corbicula cf. fluminea (O. F. Müller, 1774), was present in the modern sample; its absence from the 1926–1927 samples is not surprising given its relatively recent introduction to North America. Four of five larger gastropod species were not found in 2020–2021, but due to low abundances in 1926–1927, their 2020–2021 absences are not statistically significant. We discuss faunal changes with respect to increased human activity and introduced crayfish and snail-eating fish. Additional survey work could address whether the larger species persist in low numbers.