The Floods of 1993
The Floods of 1993Long before Iowa's flooding rivers reached their crest, several CGRER members were designing research to evaluate the floods' effects and to link flooding to events past and future. The researchers' readiness to do so was not unusual; concern with global change invariably involves climate, and the pulse of the earth's water constitutes a major element of the climate regime. So when the heavens, as if in gift, released their bounty this past summer at the doorsteps of CGRER's researchers, they willingly stepped out and into the flow.
Rivers carry much more than water. Last summer, they moved refrigerators, fledging birds, pigs, and oil tanks, among other things. And always they move sediment - small particles of soil and rock, sometimes absorbed pesticides or other toxic chemicals. Geographer Rajagopal, a University of Iowa water resources specialist, collected and analyzed over 200 eastern Iowa water samples during the height of flooding this past July and August. He questioned the conventional assumption that copious flood waters would dilute the usual concentations of water-borne pesticides. And indeed he found that the herbicides atrazine and alachlor were at annual maximum concentration during peak-flow period.
R. Rajagopal taking a water sample.During those 30 to 40 days, Raj extrapolated that the total load of agrochemicals from Iowa fields into the Mississippi was as high as the normal annual load of other years. In drier years, agricultural chemicals remain in place in the upper few feet of soil. But water flowing over and through last summer's saturated soils was pushing pesticides directly into the rivers. This led to as much as 1,000 pounds of alachlor and 7,500 pounds of atrazine washing down the Iowa River through Iowa City during the 40-day peak flow - and an estimated total of 20 metric tons of alachlor and 175 metric tons of atrazine (amounts exceeding the normal annual levels) flowing down the Mississippi and into the Gulf of Mexico. The ecological implications of such short-term shock releases of toxic chemicals remain unknown. Following the peak floods, herbicide levels in water samples decreased rapidly.
Now that flood waters have receded, University of Iowa geomorphologist Frank Weirich is examining the sediments they left behind. He has been repeating pre-flood studies that outlined the sediment deposits at the bottom of the Coralville Reservoir as well as those on the Cedar and Iowa River floodplains. By comparing pre- and post-flood deposition patterns, he will be able to quantify the amount of sediment moved by the 1993 floods and explain how the floods scoured sediment from some areas and left it elsewhere, information critical to managing Iowa's rivers and reservoirs. Ultimately, he also to predict the sediment deposition patterns of future floods. Such predictions will contribute to efforts for conserving Iowa's topsoil, maintaining the water-holding capacity of reservoirs, and managing river-associated features such as levees, sediment-free navigation channels, and wetland wildlife habitat - features important to sustaining Iowa's river systems economically and environmentally.
Frank's readiness to analyze the 1993 midwestern floods springs
from his decade of designing and observing "controlled floods"
in various types of "outdoor laboratories" in southern
California and elsewhere around the world.
After collecting preliminary data in a river valley, he releases
specific quantities of sediment-laden water from an upstream
reservoir, in effect simulating a natural flood.
Frank then can return to the surveyed valley to record the
depth and pattern of sediment deposits and to study how they
have been influenced by floodplain vegetation, river channel
shape, and the like.
He emphasizes that the new technologies
available through computer-based Geographical Information
Systems (GIS) have multiplied the power of his field studies.
GIS computer simulations transform linear data sets into
three-dimensional maps of river basins and lake bottoms,
which can be extended forward in time to predict how rivers
and reservoirs will function in the future.
Although the 1993 floods were the greatest in historic record, similar large-scale climatic events have defined the landscape's past. CGRER member Luis Gonzalez, along with fellow geologist Mark Reagan, obtained a CGRER seed grant in 1992 to test whether stalagmites could be used to identify and date major climatic events in the Midwest. His research confirmed that analysis of the growth rates and atomic composition of these cave structures could indeed provide a detailed record (accurate to the decade, perhaps even to the year) of precipitation, temperature, and vegetation over the past 7,000 to 10,000 years.
That seed grant provided the documentation needed for the researchers to receive large grants from the National Science Foundation and National Oceanic and Atmospheric Administration, to develop further their definitions of prehistoric midwestern climates and ecosystems by using stalagmites. (CGRER geologist Richard Baker is also involved in the newly-funded studies.) The results of these new studies will prove interesting not only to academics. The validation of complex climate models being developed to describe future global warming requires information about prehistoric climates, and other techniques for developing this information are far less detailed and accurate. Also, the floods of 1993 and other climatic anomalies can best be understood if analyzed in light of long-term midwestern climatic patterns and trends. By placing last summer's floods in perspective, examination of cave stalagmites may significantly assist researchers in predicting Iowa's future climates.
How can the negative aspects of flooding be reduced? Hydrometeorologist Konstantine Georgakakos, a research engineer with the Iowa Institute of Hydraulic Research, is hoping that improved use of Iowa's large reservoirs, could pose some answers. A month after the severe flooding commenced, he received a National Science Foundation grant to determine this century's frequency of floods of similar magnitude and extent in Iowa. A detailed analysis will be accomplished using computer models that link the degree of soil saturation to rainfall. The models then will tie predictions of weather and flooding to operating procedures for Iowa's reservoirs. The use of computer-based mathematical models to determine the timing of water release from a reservoir should dramatically decrease the likelihood of downstream flooding. This new study plays off of Konstantine's earlier work on hydrologic simulation and weather prediction in the upper Des Moines River basin, including investigations of the relationship between soil moisture and climate funded by a CGRER seed grant in 1992.
Taking a different tack, Iowa State University's Eugene Takle, a professor of atmospheric science and agronomy, ventures that his efforts with weather prediction will allow communities to prepare better for approaching severe weather. Current National Weather Service forecasting models predict only approximate rainfall for broad areas. Gene's tests of a sophisticated fine-resolution model will determine its ability to predict midwestern rainfall to the extent, for example, that the amount of water to fall on an entire drainage basin could be accurately foretold by as much as a day. Such forecasts could improve the planning of crucial control operations such as levee management and sandbagging.
Although theories about 1993's widespread flooding wafted through the Midwest's moist air this past summer, no one was willing to hazard a declaration concerning the flood's causes, much less venture whether these waters were part of nature's variability or human-induced alterations. But the summer's driving rains pounded one point home: short-term climatic variations can shock us into different uses of the land and attitudes toward our earth. The research of CGRER members already is shaping these emerging uses and attitudes by concretely guiding how Iowa and Iowans will cope with major floods in the future.