Submitted to: Catena
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/25/2006
Publication Date: 1/25/2007
Citation: Benedetti, M.M., Daniels, J.M., Ritchie, J.C. 2007. Predicting vertical accreation rates at an archaeological site on the Mississippi River floodplain: Effigy Mounts National Monument, Iowa. Catena. 69(2):134-149. Interpretive Summary: This study demonstrates the difficulties inherent in trying to predict future development of a large, complex alluvial floodplain. Two independent methods suggest that the Sny Magill Effigy mounds may begin to be buried by vertical accretion within 150-300 years. While the convergence of these two estimates is encouraging, they are both predicated on current flood frequency and sediment regimes of the Upper Mississippi River. As long as the conditions that have prevailed over the past 40-100 years continue, these predictions will be fairly legitimate. Given predictions of global warming, and ongoing land-use change in the watershed, the assumptions behind these predictions will likely be invalidated before the end of the 21st Century. Of particular concern is the interplay of two opposing trends, increasing flood frequency and decreasing suspended load. Recent studies suggest that large floods have been more frequent since about 1950. The massive floods of 1965, 1993, and 2001, for example, each attained discharges that might be expected once every 100 years on various parts of the Upper Mississippi River. On the other hand, several recent studies suggest that accretion rates on the Upper Mississippi River floodplain have decreased in recent decades, mainly as a result of soil conservation practices that produce lower sediment concentrations. This study illustrates some of the properties of geomorphic systems that make it difficult to predict their future evolution. These difficulties are largely the result of complex response due to spatial and temporal lags, and the effects of time scale on measurement of process rates. The geomorphic evolution of large rivers integrates environmental change in the watershed over multiple spatial and temporal scales, making it difficult to predict the response to a given perturbation. At this time, we lack sufficient understanding of the links between floods, land use change, and fluvial sediment transport to integrate the environmental changes in the Upper Mississippi valley into meaningful predictions of floodplain development beyond the next few decades.
Technical Abstract: The Sny Magill Unit of Effigy Mounds National Monument, Iowa, contains the largest cluster of prehistoric effigy mounds on public land in North America. The mounds are situated atop a low terrace of the Upper Mississippi River, where they are slowly being buried by overbank deposition during floods. The terrace surface includes forest soils with argillic (Bt) or cambic (Bw) horizons developed in up to 1 m of loamy overbank deposits on top of Pleistocene sand and gravel. Radiocarbon evidence suggests that the overbank deposits have accumulated since the end of the mound-building period (about 700 years BP), yielding a vertical accretion rate of about 0.6 mm/yr. On the basis of Cs-137 analysis, accretion rates over the past 40-50 years average 1.25-2.07 mm/yr, with some evidence for a decreasing rate since 1964. If these accretion rates are projected forward, several of the effigy mounds could be buried by flood deposits within 150-300 years. This Cs-137 derived estimate agrees closely with an estimate of burial times based on flood frequency and observed flood deposit thickness during recent floods. However, the floodplain and backwater environments of the Upper Mississippi River are aggrading much more rapidly than the Sny Magill terrace surface, suggesting that burial of the entire terrace could occur within 80-400 years and the entire mound group could be buried within 150-850 years. The projected accretion rates and time to burial are subject to large uncertainties because of environmental change in the watershed, including recent trends toward increasing flood stages and decreasing suspended sediment loads.