Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley

Abstract

Agricultural landscapes are more sensitive to climatic variability than natural landscapes because tillage and grazing typically reduce water infiltration and increase rates and magnitudes of surface runoff. This paper evaluates how agricultural land use influenced the relative responsiveness of floods, erosion, and sedimentation to extreme and nonextreme hydrologic activity occurring in watersheds of the Upper Mississippi Valley. Temporally overlapping stratigraphic and historical instrumental records from southwestern Wisconsin and northwestern Illinois show how agricultural modification of a natural prairie and forest land cover affected the behavior of floods and sedimentation during the last two centuries. For comparison, pre-agriculture Holocene alluvial sediments document the sensitivity of floods and alluvial activity to climate change prior to significant human influences on the natural land cover. High-resolution floodplain stratigraphy of the last two centuries shows that accelerated runoff associated with agricultural land use has increased the magnitudes of floods across a wide range of recurrence frequencies. The stratigraphic record also shows that large floods have been particularly important to the movement and storage of sediment in the floodplains of the Upper Mississippi Valley. Comparison of floodplain alluvial sequences in watersheds ranging in scale from headwater tributaries to the main valley Mississippi River demonstrates that land use changes triggered hydrologic responses that were transmitted nearly simultaneously to all watershed scales. In turn, flood-driven hydraulic adjustments in channel and floodplain morphologies contributed to feedback effects that caused scale-dependent long-term lag responses. There has been a general reduction in magnitudes of flooding, erosion, and sedimentation since the mid-20th century, largely in response to better land conservation practices. The reduction trend is most apparent on tributary watersheds of a few hundred square kilometers and smaller sizes. However, the main-channel Upper Mississippi River, with associated drainage areas between about 100,000–200,000 km 2, has experienced increased occurrences of large floods during the second half of the 20th century. Most of these large floods have been associated with snowmelt runoff which is occurring more rapidly and earlier in the season in response to a trend toward warmer winters and springs in the late 20th century. Modification of the natural drainage network through establishment of drainage tiles and channelization has also continued during the late 20th century. Tiling and channelization have increased drainage efficiency and probably have contributed in part to the occurrence of large floods on the Mississippi River, but the magnitudes of their effects are unknown at present. In spite of reduced sediment loads since about 1950 on all watershed scales, the anomalous high frequency of large floods on the Upper Mississippi River continues the accelerated delivery of agriculturally-related sediment to floodplain and backwater environments. The results of this study indicate that agricultural land use has escalated landscape sensitivity to such a degree that modern process rates provide a very distorted representation of process rates that occurred in the geologic past prior to human disturbance.