A Sedimentological Explanation for the Distribution of Archaeological Sites in a Meander Belt as Stated by the "Relict Channel Rule"

HEINRICH, PAUL V., U.S. Army Corps of Engineers, Baton Rouge, LA

Archaeological surveys performed for the U.S. Army Corps of Engineers within the lower Mississippi River and Red River Valleys have noted that archaeological deposits found on the natural levees of relict river channels consist only of surficial sites that postdate the abandonment of the associated river channel. From the results of these surveys, Richard Weinstein and David Kelley proposed the "Relict Channel Rule" in 1989. Their rule states that the archaeological deposits associated with the natural levees of an abandoned river channel will consist only of surficial sites that postdate the abandonment of the river channel.

The Relict Channel Rule has been explained as the result of preferences by prehistoric cultures to avoid the occupation of

natural levees of active river channels. Archaeologists have suggested that the lack of rich biotic resources associated with an active channel and the hazards created by periodic flooding caused prehistoric cultures to avoid settling the natural levees of active river channels. Rather, it has been proposed that prehistoric cultures settled around oxbow lakes within abandoned channel segments. In addition, archaeological studies of the Red River within Arkansas have speculated that the danger posed by rapidly eroding cut banks was another factor that discouraged the settlement of the natural levees of active river channels.
Sedimentological processes provide an alternative explanation for the Relict Channel Rule. While active, a typical meandering river channel rapidly migrates back and forth across its meander belt. During this time, its channel would rapidly migrate away from any archaeological deposits that formed adjacent to an active point bar. Simultaneously, overbank processes would quickly bury them. Also, an actively laterally migrating channel would consume the sites that form on the natural levee of a rapidly migrating cutbank. If a river cutbank were to migrate up to and stop at a preexisting site, that site would by that time be buried beneath natural levee deposits. Finally, archaeological sites formed during the initial establishment of a river course would eventually be either deeply buried y aggradation of natural levees or destroyed by lateral migration. As a result, only those archaeological deposits that date to a few tens of years prior to and postdate the abandonment of the channel will occur as surface sites. Therefore, regardless of whether the natural levees of a channel of an actively meandering river were used before or after its abandonment, the rapid lateral migration of its channel while active will produce the distribution of surface sites noted by the Relict Channel Rule.

AAPG Search and Discovery Article #91006 © 1991 GCAGS and GC-SEPM Meeting, Houston, Texas, October 16-18, 1991 (2009)