Many impacts on the environment were difficult to measure, some remain unmeasured, and many are tertiary and will take many more years before they are fully evident. The 1993 flood sizably altered the natural ecosystem of the Upper Mississippi and Missouri Rivers and their floodplains, changing many environmental conditions forever. The 1997 flooding also did enormous environmental damage in the floodplains in Kentucky, Ohio, Indiana, and Illinois. The 1996 flood did only minimal environmental damage with soil erosion and pollution of surface waters being the biggest impacts. Along with the flooding and related excessive erosion came further erosion and extensive silting to the floodplains and their wetlands. Although the two big floods (1993 and 1997) damaged some trees and plants, they generally provided a windfall for most plant and animal species, especially fish populations. Prolonged immersion of the nonfarmed portions of the flood-plains had deleterious effects on certain trees. When levee breaks suddenly inundated vast areas, some wildlife already isolated by the flood drowned. Populations of certain insect pests were altered, at least on a 1-year time scale.
With more than 1000 levees failing in the Midwest in 1993, turbid and sedimentladen water moved out of the river into the newly exposed floodplains. Many backwater lakes along the Mississippi and Missouri Rivers had already lost between 70 and 100% of their capacities and lost substantially more volume after receiving sizable quantities of sediment during the 1993 flood. These excessive silts and sand in the floodplains smothered vegetation and compromised large areas of productive farmland.
Floodwaters with high flow rates resulted in much-above-normal amounts of eroded sediments and agricultural chemicals in the rivers, and these impacts from all three floods were sizable on water quality. The daily load of atrazine passing in the Mississippi River near Cairo, Illinois, in July 1993 was 12,0001b per day, four times higher than during any previous year. A large percentage of the eroded herbicides and nitrates entering Midwestern rivers was carried into the Gulf and had a major impact on the ecosystem of the Gulf shore area. Another water quality problem in 1993, 1996, and 1997 along the rivers was large amounts of raw sewage, bacteria, viruses, and parasites carried by the floodwaters. Health officials were concerned that the organisms in the water could cause hepatitis, cholera, typhoid, or gastrointestinal illnesses; therefore, thousands of persons living and working along the river were inoculated to prevent disease outbreaks and, fortunately, waterborne diseases were minimal in all three floods.
Many parts of the ecosystems in and around the flooded rivers of 1993 and 1997 derived benefits from the floods. Large river floodplain ecosystems in the river system have adapted to exploit seasonal flooding. A major problem in the 1993 flood related to Zebra mussels, which were inadvertently introduced in 1986 to the Great Lakes. The mussels entered the Illinois River from Lake Michigan via the canal system at Chicago in 1991 to 1992 and established themselves in the Upper Illinois River. These mussels released their larvae as the 1993 flood was occurring, and the floodwaters transported huge numbers of the larvae into the lower Illinois River and downstream into the Mississippi, moving laterally into many floodplain lakes and up many tributaries, and into industrial and municipal treatment plants. The Zebra mussel has prospered in its newly colonized habitats, adding greatly to the cost of water treatment and plant maintenance, jeopardizing the survival of native mollusks, and even altering river food webs by filtering detritus, suspended sediment, and the contaminants associated with these particles. This flood-induced spread of Zebra mussels was truly an "environmental disaster."
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