At hundreds of sites throughout the US, native peoples were interred in special burial mounds. Those at Effigy Mounds National Monument include conical and animal-shaped mounds, including these at the Sny Magill site in Clayton County. They overlook Johnson’s Slough, a backwater wetland associated with the Mississippi River.
According to a study by Benedetti and others published in 2007, the mounds are disappearing, being covered in sediments being deposited when the river rises high enough to flood this area. These ancient mounds are predicted to disappear, some in as little as 80 years, although it could take up ten times that long to bury the whole mound group. The National Park Service representative at the Visitor’s Center told me an initial plan to preserve the mounds had failed, but that a new plan was in the works.
Justifying the work, and developing a suitable plan, must include the estimation of accretion rates. In other words, we need to understand the speed and specific mechanisms of the process of burial by river sediments. A useful approach, employed in the 2007 study, is examining sediments above and below a time marker (horizon) deposited at a known time period. These researchers were able to precisely pinpoint the sediments laid down between 1950 and 1963.
“But Paul,” you say. “What is the ‘secret sauce’ in that particular layer of sediment?”
Thoughtful Cold Warriors exploded atomic bombs out in the open (so-called atmospheric testing) in great numbers during those 13 years, spreading a radioactive dust; the dust then acts as a sort of marker that shows up everywhere, including this floodplain of the Mississippi River. Cesium-137 in the fallout was used by these researchers (other studies use other isotopes). Sediments without that isotope were laid down before or after ’50-’63, with the older sediments below the radioactive layer and the newer sediments above it.
Whatever plan is developed to save the mounds, it must contend with two profound facts: the river is mighty, and it is changing. A glance around the site shows all the signs of water and mud and flotsam scattered about when the Mississippi is high. We are, after all, in the floodplain of one of the world’s great rivers. But the other fact is that the river no longer functions as it once did. We have changed the drainage of water into the river (by altered land-use all around the region), and we have altered the river channel itself (through various control structures and regulation). We’ll examine more about river dynamics in other 99wetlands posts. In the meantime, just remember that these floodplain wetlands are still here and still conversing with the Great Water, as the people in those mounds no doubt observed so long ago.