Environmental-flow standards are needed to maintain hydrologic regimes that can provide optimal physical habitat to support the outstanding freshwater biodiversity of rivers in the southeastern US. Here, we used the physical habitat simulation (PHABSIM) model to estimate habitat-based environmental flows for 5 river reaches across 4 physiographic provinces of Alabama, using data from published habitat suitability curves (HSCs). We found that physical habitat-flow relationships, although varying by target species and local reach characteristics, are related broadly to river size and channel geometry. For the study rivers, optimal habitat was maximized at low levels of flow on smaller rivers in the Appalachian Plateau and Piedmont regions, while suitable habitat continued increasing with increased discharge for larger rivers on the Coastal Plain.