Petrochelidon pyrrhonota (Cliff Swallow) initially expanded its breeding range to the Great Pee Dee River corridor of the Carolinas in the early 1980s. In 2012, I documented approximately 1700 active nests of the Cliff Swallow at 20 colony sites in the Lower Piedmont of North Carolina and Coastal Plain of South Carolina along this river corridor. Cliff Swallows nested primarily at bridges and other water-based sites but also bred at several land-based sites, including the first colony documented at a highway overpass in the Carolinas. Compared to colony sizes in the early 1980s and mid-1990s at 3 major sites in North Carolina, numbers in 2012 were approximately 1500% and 400% higher, respectively. Before this study, Cliff Swallows nesting along the Great Pee Dee River was documented only as far as Cheraw, at the Fall Line. This study demonstrates that breeding now occurs 145 km farther south along this corridor at 6 sites, including 2 sites in the tidewater region of South Carolina. Several bridges along this corridor with favorable structural characteristics for breeding are still unoccupied. The breeding range expansion front of interior populations along and near the Great Pee Dee River has moved slowly (≈3.25 km/yr), in an apparently stepwise manner. However, the gap between expanding interior and coastal populations is confounding the pattern of range expansion in the Lower Coastal Plain of South Carolina.