Agricultural influence on landscape sensitivity in the Upper Mississippi River Valley

Agricultural landscapes are more sensitive to climatic variability than nat ural landscapes because tillage and grazing typically reduce water infiltra tion and increase rates and magnitudes of surface runoff. This paper evalua tes how agricultural land use influenced the relative responsive ness of fl oods, erosion, and sedimentation to extreme and nonextreme hydrologic activ ity occurring in watersheds of the Upper Mississippi Valley. Temporally ove rlapping stratigraphic and historical instrumental records from southwester n Wisconsin and northwestern Illinois show how agricultural modification of a natural prairie and forest land cover affected the behavior of floods an d sedimentation during the last two centuries. For comparison, pre-agricult ure Holocene alluvial sediments document the sensitivity of floods and allu vial activity to climate change prior to significant human influences on th e natural land cover. High-resolution floodplain stratigraphy of the last t wo centuries shows that accelerated runoff associated with agricultural lan d use has increased the magnitudes of floods across a wide range of recurre nce 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 allu vial sequences in watersheds ranging in scale from headwater tributaries to the main valley Mississippi River demonstrates that land use changes trigg ered hydrologic responses that were transmitted nearly simultaneously to al l watershed scales. In turn, flood-driven hydraulic adjustments in channel and floodplain morphologies contributed to feedback effects that caused sca le-dependent long-term lag responses. There has been a general reduction in magnitudes of flooding, erosion, and sedimentation since the mid-20th cent ury, largely in response to better land conservation practices. The reducti on trend is most apparent on tributary watersheds of a few hundred square k ilometers and smaller sizes. However, the main-channel Upper Mississippi Ri ver, with associated drainage areas between about 100,000-200,000 km(2), ha s experienced increased occurrences of large floods during the second half of the 20th century. Most of these large floods have been associated with s nowmelt 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 cen tury. Modification of the natural drainage network through establishment of drainage riles and channelization has also continued during the late 20th century. Tiling and channelization have increased drainage efficiency and p robably have contributed in part to the occurrence of large floods on the M ississippi River, but the magnitudes of their effects are unknown at presen t. In spite of reduced sediment loads since about 1950 on all watershed sca les, 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 indica te that agricultural land use has escalated landscape sensitivity to such a degree that modem process rates provide a very distorted representation of process rates that occurred in the geologic past prior to human disturbanc e. (C) 2001 Elsevier Science B.V. All rights reserved.