Recent studies on Emydid turtles demonstrate a relationship between shell shape and environmental variation. Turtles in fast-flowing riverine habitats have repeatedly been shown to have flatter shells than those from habitats with little or no water flow, such as ponds or slow-moving rivers. These results are consistent with body shape patterns observed in fishes and other aquatic organisms and hypothesized to be a consequence of local adaptation. Missing are studies with turtles that explicitly address this adaptive hypothesis, especially given the distinguishing characteristic of the turtle shell in that it does not function in locomotion, is a rigid structure, and may represent additional reproductive constraints for flatter turtles. The stripe-necked musk turtle (Sternotherus peltifer) inhabits a wide variety of stream habitats throughout its range in the Southeastern United States. Despite its varied habitats, the pattern of shell shape variation has not yet been documented in this or any other bottom-walking turtle species. We used 3D geometric morphometrics to assess shell shape variation in S. peltifer from the Cahaba River drainage of central Alabama. The Cahaba drainage is bisected by a fall line, where swift Appalachian streams shift to the slower rivers of the coastal plain. We found an ecomorphological pattern consistent with previous studies on other turtle species. In light of this pattern, we examined clutch size data to determine whether there is a trade-off between shell shape and reproductive output, which would suggest that flatter shells are a local adaptation to fast-flowing environments.