Post Wisconsin alluvial stratigraphy of the mainstream Columbia River drainage developed by alternating cycles of alluvial deposition and landform stability. Holocene aggradation episodes followed closely behind the Upper-most Pleistocene catastrophic (Glacial Lake Columbia) floods dated in this study to 12,800+/-50 BP. Initiation of a multiple Holocene fine-grained terrace system commenced prior to 9000 BP and continued through to approximately 500BP. A comparison of stratigraphic sequences reveals that (1) extensive tracts of early Holocene floodplain are preserved along the mainstream Columbia River; (2) there is little variability in the age and character of the early Holocene terrace deposits; (3) sedimentologic and pedologic details of the early Holocene terrace are readily differentiated from later Holocene terraces; (4) early Holocene archaeological sites are commonly buried within the former floodplain. Details of the timing and stratigraphic characteristics of the earliest Holocene alluvial floodplain are presented.