INTERPRETING FLOODPLAIN SEDIMENTATION RATES: EXAMPLES FROM THE UPPER MISSISSIPPI RIVER SYSTEM

Radiocarbon ages for organics in upper Mississippi River (UMR) alluvium indicate that the average, millennial scale, late Holocene rates of floodplain vertical aggradation have varied between about 0.07 and 0.20 cm per year. In contrast, the average Holocene rate of vertical accretion on floodplains of small UMR tributaries has been about 0.02 cm per year. The differential between the two environments is partly due to a tendency for increasing vertical magnitudes of sedimentary facies with increasing size of the contributing watershed, but also results from continuing very high magnitude UMR influxes of tributary sediments that were previously stored during regional glacial climates. The presence of weak A/C soil horizons as well as discrete multi-centimeter thick sandy deposits associated with former large floods, indicate that average vertical accretion rates do not properly represent the more typical episodic nature of the floodplain evolution. Initial floodplain construction by lateral accretion deposition associated with point bar and other bar forms appears to have dominated both the UMR and its tributaries, although some local sites on the UMR floodplain have been dominated by progressive long-term vertical accretion. The relative thickness of vertical accretion sedimentation is both a function of the length of time that deposition has been occurring and the relative elevation of the alluvial surface. A relationship between vertical thickness of Holocene alluvium and maximum floodplain age for tributary sites indicates a strong exponential decay in rate of vertical accretion with age during the first one or two millennia and then gradually slowing with further time. Accelerated floodplain vertical accretion since the introduction of Euro-American agriculture somewhat replicates this process as very high rates of sedimentation have occurred on former relatively low floodplain surfaces while former relatively high floodplain surfaces have been characterized by low rates of sedimentation. This latter process has had a net leveling effect on former natural floodplains and has decreased the natural diversity of these alluvial environments. This research was supported by the U.S. NSF (ATM-0112614).