Fifteen alluvial sequences in Texas and Oklahoma exhibit the same late Holocene record of channel trenching at 1 ka. The erosion was preceded by slow alluvial sedimentation in most stream valleys, resulting in the formation of a cumulic, organic-rich flood-plain soil previously named the Copan Soil. The soil formed during a period of regionally moister climate, as indicated by pollen spectra, molluscan faunas, vertebrate faunas, sedimentary structures, and high alluvial water tables. At 1 ka, the regional climate changed from moist to dry, coinciding with an episode of channel incision of valley floors throughout the southern Great Plains. Channel trenching occurred simultaneously in both small and large streams in drainage basins of the Arkansas, Red, Trinity, Brazos, and Colorado rivers; the sequences are the first documented example of widespread Holocene incision accompanied by firm evidence for a synchronous change in regional climate.