We use unique fluvial gravel deposits preserved atop a regional drainage divide to confirm the role of stream capture in driving ∼250 m of incision in the transient Roanoke River basin of the Appalachian Mountains (United States). Gravel provenance constrains the pre-capture position of the divide, indicating that ∼225 km2 of basin area were abruptly connected to the base level of the capturing stream. The resulting wave of incision is currently manifest as major knickzones separating adjusting reaches from relict headwaters resembling streams of the New River basin, from which the Roanoke River was captured. The unusual preservation of the unconsolidated gravels on small relict surfaces adjacent to bedrock gorges indicates extreme spatial variability in erosion rates within the Roanoke basin, which is the first documented example of a transient passive margin basin connected to a capture event by stranded fluvial debris. Our results show the potential for stream capture across an asymmetric drainage divide to drive major transient incision independent of external forcings, such as climate change or tectonic uplift. A continuation of this process will lead to eventual capture of ∼7000 km2 of the New River basin in the relatively near geologic future.