Low frequency but high intensity events, such as severe floods or droughts, may have long-lasting effects on the structure and function of lotic ecosystems, as well as on man-made structures and human uses of rivers, floodplains, and estuaries. In the summer of 1993, major, global-scale atmospheric circulation anomalies caused unusual precipitation in the upper midwestern part of the US (return frequencies ranged from 75 to 300 years, dependent on location), creating extreme flooding (100-500-year) along the upper Mississippi River, the Illinois River, and the lower Missouri River, and many smaller tributaries US Army Corps of Engineers, 1995). In the same year and early in 1994 there also were record floods in many of the large rivers of western Europe. A symposium in August 1995 at the annual meeting of the Ecological Society of America in Snowbird, Utah, brought together researchers who either had ecological studies on-going when extreme hydrologic events occurred or responded to an event. Of the eight papers presented at the symposium, four were submitted for this volume. Two papers not presented at the symposium are included here because they are relevant to the topic of extreme hydrologic events van Oorschot et al., submitted).
The study areas in these papers are rivers, floodplains, or a receiving basin-all subject to flooding. However, the areal extent and frequency of flooding have been changed by human alterations of the catchments, floodplains, and the rivers themselves. In one case, the Loire River at Decize in France, flooding has been reduced by upstream dams and by gravel mining in the river bed, creating man-made 'drought' on the floodplain. In comparison to an adjacent, undammed reach of the Allier at Apremont, reduced flooding on the Loire reduces nutrient inputs and nutrient pools on the floodplain, causing reductions both in nutrient cycling rates and accumulation of soil organic matter (van Oorschot et al., submitted).
Seasonal flooding still occurs in the upper Mississippi River, upstream from the confluence with the Ohio River, and on a major tributary, the Illinois, which joins the Mississippi near St. Louis. In fact, both flood heights and frequency are increasing, in response to the human alterations mentioned above and to long-term cycles and trends in rainfall . Navigation dams on the Mississippi upstream from St. Louis and on the Illinois do not stop floods, but do maintain water depths for navigation during the low flow season; in effect, permanently inundating portions of the floodplain by not allowing the rivers to drop as low as they once did. Downstream from St. Louis, the river is not dammed. Approximately 70% of the original floodplain area in Minnesota and Wisconsin is unleveed and subject to overflow by the river. The proportion of the floodplain that remains