Sternotherus depressus (Flattened Musk Turtle) is a federally threatened species endemic to the Black Warrior River drainage in north-central Alabama. Individuals of both stream-dwelling and impoundment-dwelling populations were trapped for comparative demographic analyses. Carapace length was significantly longer for reservoir turtles than for stream turtles. Size-class distributions between the two populations were also significantly different, with reservoir turtle collections biased toward the larger size classes, and presumably older age classes. These results suggest a reduced recruitment in the reservoir population, and raise concerns about long-term population sustainability in impoundment habitats.