UPPER MISSISSIPPI RIVER VALLEY SEDIMENTARY ARCHITECTURE AND THE ARCHAEOLOGICAL RECORD

The alluvial fill of the Upper Mississippi River Valley records a number of significant changes in the dynamics of the main valley and its tributaries that impact our understanding of the archaeological record. The early Holocene Mississippi River underwent a series of avulsions that left abandoned channel belts along both valley margins as it changed from a bed load to a mixed load stream. By the middle Holocene the river had attained an island braided pattern in a central valley channel belt that, except at major tributary junctions, it has occupied since that time. A natural levee began to form along the eastern margin of the channel belt about 7000 B.P. and continued to develop into the late Holocene. Throughout the Holocene, alluvial fans emanating from small and moderate-size tributary valleys prograded the abandoned channel belts on either margin of the valley, and formed a dynamic mosaic of diverse environments. As the sedimentary architecture of the valley evolved landscape characteristics important to humans, such as height above flood, proximity to river channels, size, permanence, and abundance of oxbow lakes and wetlands, and the distribution of edge environments also underwent change. Large valleys are a challenge to geoarchaeologists and archaeologists because of the shear volume and depth of buried, potentially habitable landscapes, and the interaction of many sedimentary subsystems in their history. These valleys' alluvial architecture, placed in a temporal and environmental framework, is critical to identifying appropriate archaeological testing methods and for evaluating what impact earth system filters have had on the archaeological record. I discuss a stratification of the valley landscape based on burial potential and edge environments for the late prehistoric Middle Woodland Period, and differential preservation of the Early Archaic archaeological record across the valley floor and in different valley reaches.