RATE, MAGNITUDE AND IMPACT OF LEGACY SEDIMENT ACCUMULATION ON A HEADWATERS WATERSHED IN EASTERN IOWA

Human-induced sedimentation affects floodplains throughout the world and is especially pronounced in watersheds under agricultural land use. Previous studies in the upper Midwest have documented the widespread occurrence of legacy sediment (post settlement alluvium) and have concluded that much of this sediment accumulated soon after agricultural disturbance of the watershed. Few studies have focused on the chronology of the legacy sediment package, or on the impacts the sedimentation had in the functioning of the agricultural floodplain. We use an intensive coring program, ²¹⁰Pb and ¹³⁷Cs dating, fly ash occurrence, and sedimentological reconstructions to: 1) understand the age structure of the legacy sediment, 2) determine the source of the sediment (directly from adjacent slopes, overbank during floods) across the floodplain, and 3) reconstruct the evolution of this floodplain during the Anthropocene. Our reconstructions indicate that although floodplain sedimentation did begin soon after agricultural land use began, significant sedimentation continued to occur after the channel entrenched and began to contain small-to-moderate floods, and also subsequent to establishment of extensive soil conservation measures in the 1960’s.

The earliest legacy sediment began to accumulate between 1860 and 1880 on a poorly drained, un-channeled floodplain with little relief. As soil erosion and runoff from watershed slopes increased, flood magnitude and stream power increased and a permanent channel formed and incised the floodplain. Over the past century and a half floodplain elevation has increased, the stream has continued to incise and widen, and floodplain soils have become more well drained. These changes have had significant effects on the ecological function of the floodplain, changing it from a zone dominated by transformations of sediment, organic matter and nutrients to a zone where these materials are transported to the stream network.