Stream Channel Change in an Intensively Managed Agricultural Landscape: Implications for Critical Zone Processes

Abstract:

During the Anthropocene, humans have had a substantial impact on fluvial systems throughout the world. Perhaps nowhere is the human imprint on stream systems more pronounced than in the intensively managed agriculture landscapes of the midwestern United States. This study examines changes in the structure of the stream network and in the planform dynamics of stream channels within the upper Sangamon River basin in Illinois – a watershed that is part of a new Critical Zone Observatory focusing on intensively managed landscapes (IML-CZO). The research explores changes in network structure as the landscape changed from prairie and forest into drained farmland dominated by row crop agriculture. It also documents the planform dynamics of stream and river channels over the past 80 to 100 years. Results show that the spatial extent of channels within the stream network expanded greatly as land was cleared and drained for agriculture. Expansion of the network into headwater portions of the watershed occurred through the construction of drainage ditches that serve as outlets for tile drainage systems underlying relatively flat, poorly drained farmland. Analysis of planform dynamics reveals that most of these drainage ditches have not changed alignment since initial construction. Although drainage ditches are maintained by local drainage districts, these human-created channels also are remarkably resistant to change in planform over time. The major type of planform change in headwater streams involves artificial straightening of meandering channels to expand the extent of drainage channels. Many sections of the meandering Sangamon River are heavily forested and exhibit little or no planform change over the past 80-100 years. Sections that are most active tend to occur where forest cover is less prevalent due to clearing of trees for pasture or cropland. Overall, the results demonstrate the pronounced imprint of humans on the structure and planform dynamics of a fluvial system in an intensively managed watershed. Undoubtedly this imprint has an important influence on downstream fluxes of water, nutrients, and sediment, which are a primary focus of ongoing process-based research in the IML-CZO.