Spatial distributions and movements of aquatic animals are typically defined by the dimensions of the aquatic system in which they live. Aquatic turtles often confound such definitions with terrestrial movements, and movements within the aquatic system remain largely undescribed. We studied the movements of adult female painted turtles (Chrysemys picta) and red-eared sliders (Trachemys scripta) in Reelfoot Lake, a large aquatic system. We determined the distance moved, home-range size, and habitat use to investigate interspecific variation, as well as the influence of habitat dimensions on turtle spatial ecology. Daily movements peaked for both species between May and August. Trachemys scripta exhibited longer movements, maintained larger home ranges, and selected deeper areas of the lake than did C. picta. Compositional analysis indicated that the shoreline habitat was used most frequently by both species, relative to its availability. However, although C. picta selected the shoreline above all other habitat types, T. scripta selected the open water areas of the lake as well. Morphological differences and dietary preferences likely explain much of the interspecific variation we observed. Turtle movements and home-range sizes we report are remarkably larger than previous estimates and may be related to the abundance of suitable habitat within Reelfoot Lake. These results demonstrate that turtle spatial ecology is strongly influenced by local environmental factors and illustrate the potential pitfalls of describing “typical” movements of aquatic turtles.