Unionids grow their shells incrementally. Thus, they potentially preserve ontogenetic history and ambient environmental conditions as seasonal variation in oxygen and stable carbon isotope compositions ([delta]18O and [delta]13C, respectively). This study combined geochemistry and sclerochronology to investigate whether Lampsilis cardium precipitated its shell in isotopic equilibrium with the ambient environment and if growth bands were deposited seasonally. To characterize ambient conditions, we measured temperature, [delta]18O[subscript WATER], [delta]13C of dissolved inorganic carbon (DIC), and pH biweekly from September 2002 to July 2004 in two Iowa rivers, Boone River (BR; agricultural watershed with C-4 plants: corn) and Buffalo Creek (BC; urban watershed with C-3 plants: soybeans). These data were used to calculate predicted values of isotopic composition to compare to measured shell values. One hundred fifty-four individuals were marked at the postero-dorsal margin in September 2002 and June 2003. Seven recaptured shells collected in July 2004 were measured isotopically across three growth bands from the growing edge towards the umbo. [delta]18O[subscript SHELL] corresponded to predicted values from early spring to early fall. Predicted winter values were not recorded, representing winter growth cessation. Rapid temperature change appears to control the onset and cessation of growth. The location of seasonal growth bands corresponded to winter cessation, and non-seasonal disturbance bands were identified. In BR, [delta]13C[subscript SHELL] did not follow predicted values and were offset by +1 to +4%0 (VPDB). In BC, three of the four shells tracked predicted values during spring and summer, but not during fall. This finding suggests that vital effects control [delta]13C in some of the shells all of the time and in all of the shells some of the time. Regardless of the correspondence (or lack thereof) to predicted values, [delta]13C values of shells from BC are more negative than those from BR. This result is consistent with expected offsets between watersheds having C-4 (BR) versus C-3 vegetation (BC). We conclude that [delta]18O[subscript SHELL] can be used to decipher seasonal variation in shell growth. Although [delta]13C[subscript SHELL] may not be useful to reconstruct seasonal change in sources of DIC, it can be used to reconstruct changes in the dominant vegetation within a watershed.