TIMING AND CHARACTERISTICS OF EARLY HOLOCENE AGGRADATION, COLUMBIA RIVER, WASHINGTON STATE

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.