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Water Science and Applications



  • Paleohydrology
  • Floods

Index Terms

  • 1815 Hydrology: Erosion and sedimentation
  • 1821 Hydrology: Floods
  • 1824 Hydrology: Geomorphology
  • 1694 Global Change: Instruments and techniques



Watershed scale and the stratigraphic record of large floods

J. C. Knox and J. M. Daniels

The sedimentological characteristics of overbank alluvium deposited by large floods vary systematically with size of the contributing upstream watershed. The effects of watershed scale result from the strong association between areal extent of flood-producing meteorological events and watershed size, and from the tendency for floods to experience downstream decreasing power to transport coarse sediment. Stratigraphic evidence of large overbank floods that have occurred during historical and Holocene time in the Upper Mississippi Valley includes: (1) large clasts from which former flood depths can be estimated and (2) fine-texture (sand, silt, and clay) slackwater deposits from which flood depths can sometimes be estimated but more often provide only flood frequency information. The use of clasts to compute depths of flow competent for their transportation is mainly restricted to watersheds smaller than a few hundred km2 in the Upper Mississippi Valley, but use of slackwater sediment for flood frequency analyses extends across all sizes of watersheds including the main channel upper Mississippi River. A 1–2 cm vertical sampling resdution for grain size analyses is ordinarily sufficient for detecting sedimentological evidence of large floods, although higher resolution sampling is necessary when large floods have occurred in close temporal succession. The widespread distributions of the fine-texture slackwater sediments on floodplains and low terraces provide relatively long and nearly continuous records of large overbank floods that have occurred in the Upper Mississippi Valley. Recognition of a relatively complete record of large post-glacial floods is supported by radiocarbon ages that indicate the Holocene upper Mississippi River along western Wisconsin has been dominated by progressive aggradation. Heavy summer rainfalls are responsible for the largest floods on watersheds smaller than several hundred km2 in the Upper Mississippi Valley, but on larger watersheds, snowmelt is a principal cause of the largest floods. The scale-related different origins for large overbank floods provide a framework from which long-term seasonal behavior of large floods can be evaluated. Results show that rainfall and snowmelt floods in the region have for certain extended periods of the late-Holocene behaved in similar ways while at other times may have differed significantly. These findings indbate that more detailed comparisons of paleoflood histories from wide-ranging sizes of watersheds in the Upper Mississippi Valley can provide relatively specific information concerning how climate changes influence large snowmelt and rainfall floods. In turn, the results provide a more sound basis to foresee how magnitudes and recurrence frequencies of large floods might respond to future climatic changes.

Citation: Knox, J. C., and J. M. Daniels (2002), Watershed scale and the stratigraphic record of large floods, in Ancient Floods, Modern Hazards: Principles and Applications of Paleoflood Hydrology, Water Sci. Appl., vol. 5, edited by P. K. House et al., pp. 237–255, AGU, Washington, D. C., doi:10.1029/WS005p0237.

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