A Watershed Year: Anatomy of the Iowa Floods of 2008

2. Why Were the 2008 Floods So Large?

In June 2008, eastern Iowa experienced some of the worst flooding ever recorded . Floods, commonly defined as river waters overflowing their banks, devastated cities and the countryside alike. Some 1.2 million acres of Iowa’s agricultural land was affected by floodwaters.1 From a plane, it was difficult to decipher the main channel of many rivers. The confluence of the Iowa and Mississippi Rivers seemed like a sea dotted with silos that protruded from the water’s surface. In Cedar Rapids, a six-foot-tall man standing on the west bank levees would have had water flowing six feet above his head. The peak flow on the Cedar River at Cedar Rapids reached 140,000 cubic feet per second (cfs) on June 13. This extremely large flow was a full five times as large as the river’s average annual peak flow at this site.2 On the Iowa River at Marengo, the peak flow reached 51,000 cfs on June 12, four times as large as the average annual peak flow at this site. Downstream from Marengo, the Coralville Reservoirofferedsomeprotection,butinIowaCity(withadowntownareaabout 8 river miles below the dam), the Iowa River still reached a peak flow of 41,100 cfs and flooded significant areas of the University of Iowa campus. Peak flow there was three times the average annual peak flow (USGS 2009). 2 Why Were the 2008 Floods So Large? Witold F. Krajewski Ricardo Mantilla 20 rising rivers, spreading waters The sense of the 2008 floods’ tremendous magnitude also is portrayed in figures 2-1 and 2-2, hydrographs that plot May and June discharge (river flow) atseveralstreamgagesontheCedarandIowaRiversandtheirtributaries.Note how in most instances, especially for rivers draining large areas, the 2008 peak greatlyexceededtheaverageannualpeakflow,alevelthatroughlycorresponds to a river filling its banks to the brim. What was the genesis of these floods? Why were they so large? Many have ponderedthesequestions,tryingtograspuniquefeaturesofweatherandlandscapethatmightprovideanswers .Hereweconsiderthreecontributingfactors: the severe winter that preceded the floods, the high-intensity rainstorms of late May and early June, and the possibility of a perfect storm—not necessarily extremelylarge,butoneinwhichprecipitationwasperfectlytimedandlocated to raise the flow in river drainage networks to extraordinary levels. First, consider the preceding season. The region experienced one of the snowiestwintersinrecentmemory.AveragesnowdepthacrossIowareached11 inches by late February (NOHRSC 2008). The snowmelt saturated the ground, whichremainedwetwellintoMayandsignificantlydelayedtheplantingofcrops in many parts of the state. With cool temperatures and croplands remaining bare, little vegetation was available to dry the soil through transpiration (the pulling of moisture from plant roots through leaves into the air). Could these winter conditions help explain the June flooding? Examine the graphsinfigure2-3(alsoplate14).PanelAshowsthedailyvariationofsnowcover, snow depth, and snow water equivalent (the amount of water that would result frommeltingthesnow)forDecember2007throughMarch2008,averagedover thestateofIowa(NOHRSC2008).Notethatwhileaveragesnowdepthreached over 11 inches by the end of February, the snow was virtually gone by the end of March, with snowmelt hastened by March temperatures rising above freezing (panel B; IEM 2008). The effect of this snowmelt on March flooding is clearly seen in panels D and E. That month, river discharges at both Cedar Rapids and Marengoroseinasinglewave,reachingnearly40,000cfsand18,000cfsrespectively ,butthendroppedbacktonormallevelsbytheendof March(USGS2009). Thus, the heavy winter snows were not directly responsible for later flooding. Thesnowsdid,however,leadtoflood-proneconditions.Someofthemelting snow remained in the fields as soil moisture. Thus, when the April rainstorms arrived (panel C; IEM 2009), the soils could absorb little of the spring rainfall. Instead, the wet ground produced significant runoff and additional high discharge waves in the rivers (note April discharges in panels D and E). Flooding from these storms peaked around the end of April. FIGURE 2-1 Streamflow discharge hydrographs for several streamgage locations in the Cedar River basin through the 2008 flood period, displaying flow phenomena explained in this chapter. Locations of Iowa streamgages are shown in plate 1 and figure 2-5; Black Hawk Creek is a tributary that enters the Cedar River at Waterloo. Vertical lines, May 31–June 20, delineate individual days. Upstream drainage area in square miles follows the station name. These hydrographs show that the discharges rose throughout the basin during the flood, although to different degrees (note that the discharge scale on the left is different for every location shown): the discharges increase for locations draining larger and larger areas, with the small creek (Black Hawk Creek) showing a smaller but very sharp rise and fall. River discharges such as those displayed here reflect both precip­ itation and basin geomorphology. Illustration by Radoslaw Goska, based on USGS 2009. Discharge...