SPATIAL VARIATIONS IN THE MAGNITUDE OF THE 1993 FLOODS, RACCOON RIVERBASIN, IOWA

The persistent position of a mid-level circulation pattern in the summ er of 1993 supported the formation of frequent storms in the upper Mid west (Bell et al., 1993). Storm events that occurred during this perio d caused multiple episodes of flooding in the region (Wahl et al., 199 3). The high floods along the upper Mississippi River generated debate s about the effectiveness of flood control measures and the effects of land use on flood peak discharges. In order to examine spatial variat ions in flood peak discharges, we surveyed flood channels and flood pr ofiles on the Raccoon River and its tributaries in west-central Iowa. The Raccoon River basin has variable topography and land use, includin g some of the most intensely agricultural land in the United States. E xtensive ditch networks and subsurface tile drain systems have been in stalled to enhance runoff and accelerate drainage. We found that sites within and downstream of modified channels had higher magnitude flood s than than other comparably sized basins in the Raccoon River basin a nd the upper Midwest for which data were available. Erosion patterns a lso followed land use patterns; the upper portions of channelized Racc oon River tributaries experienced short times of flooding and had less erosion than downstream channelized reaches that were severely eroded and had significant damage to bridges and other structures.