Impact of the alluvial style on the geoarcheology of stream valleys

Abstract The extensive archeological record within river valleys provides an environment long favored for study by the archeologic community. This record of human occupation varies with the alluvial style of the river, resulting in each river valley having a unique archeological record controlled by three factors; location of occupation sites, preservation of those sites, and recognition of the sites. If problems of site preservation and recognition in a river valley can be recognized and addressed, a better insight into settlement patterns and complex human–environment interactions can result. This study represents a synthesis of extensive published and secondary data sets pertaining to archaeology and alluvial geomorphology of the lower Missouri, Red, and Mississippi Rivers, to examine the relationship of prehistoric site distribution with fluvial geomorphology. The archeologic data are from published literature and government agencies. The geomorphic data are from a variety of published and unpublished investigations, including maps of alluvial geomorphology, digital elevation models, aerial photographs, and topographic maps. The data were input into spreadsheets and a GIS for qualitative and quantitative analysis. Prehistoric human occupation of the narrow Missouri valley floor is limited, with only 0.061 recorded sites/km2. Additional sites are likely buried by thick vertical accretion sediment on the limited prehistoric portions of the floodplain and within alluvial/colluvial fans along the valley margin. Non-random preservation patterns, such as the absence of sites from the youngest cultural period, are attributed to shifts in settlement patterns. In comparison to the lower Missouri River valley, prehistoric occupation of the lower Red and Mississippi rivers was much more extensive and the density of occupation was 0.274 sites/km2 and 0.277 sites/km2 respectively. Significant differences do exist in the age of sites and their relationship to different floodplain depositional environments. Many sites along the active meander belt of the Red River were likely lost due to high rates of channel migration. Nevertheless, intense occupation of Paleo-Indian through Caddoan groups assures that many significant sites have been preserved. The wide valley and high rates of sedimentation suggest that many sites are likely buried, particularly along natural levees flanking an older meander belt. Recorded sites are also present along streams that incise the terrace, along the terrace margin, and along small channels that cross the backswamp. The spatial and temporal site distribution along the lower Mississippi valley is distinct in comparison with the Red River because the Mississippi valley is wider, the terraces are younger and less dissected, and the floodplain has more relief and is not broadly inundated by floodwater with relatively low sediment concentrations. Most of the significant Archaic through Mississippian period floodplain sites are located along the surfaces of abandoned Mississippi and distributary meander belts which aggraded above the adjacent backswamp and have rarely been influenced by flood sedimentation since abandonment. Abundant Paleo-Indian through Mississippian period sites are also widely distributed across undissected terraces. Only a few significant Mississippian sites are present along the active meander belt and in the backswamp where flooding was common. In comparison with the Red River, the archaeological record of the floodplain is less likely to be influenced by burial or erosion due to the Mississippi River having lower rates of overbank sedimentation and channel migration, respectively. The variable alluvial styles of the lower Missouri River, lower Red River, and the lower Mississippi River valleys presented varying opportunities and constraints to occupation and sustained settlement, and resulted in widely varying archaeological patterns that are partly dependent upon the alluvial geomorphology.