Disturbance and recovery of large floodplain rivers |
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Authors: | Richard E Sparks Peter B Bayley Steven L Kohler Lewis L Osborne |
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Institution: | (1) Stephen A. Forbes Biological Station, Illinois Natural History Survey, P.O. Box 599, 62644 Havana, Illinois, USA;(2) Center for Aquatic Ecology, Illinois Natural History Survey, 61820 Champaign, Illinois, USA |
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Abstract: | Disturbance in a river-floodplain system is defined as an unpredictable event that disrupts structure or function at the ecosystem,
community, or population level. Disturbance can result in species replacements or losses, or shifts of ecosystems from one
persistent condition to another. A disturbance can be a discrete event or a graded change in a controlling factor that eventually
exceeds a critical threshold.
The annual flood is the major driving variable that facilitates lateral exchanges of nutrients, organic matter, and organisms.
The annual flood is not normally considered a disturbance unless its timing or magnitude is “atypical.” The record flood of
1973 had little effect on the biota at a long-term study site on the Mississippi River, but the absence of a flood during
the 1976–1977 Midwestern drought caused short- and long-term changes. Body burdens of contaminants increased temporarily in
key species, because of increased concentration resulting from reduced dilution. Reduced runoff and sediment input improved
light penetration and increased the depth at which aquatic macrophytes could grow. Developing plant beds exerted a high degree
of biotic control and were able to persist, despite the resumption of normal floods and turbidity in subsequent years.
In contrast to the discrete event that disturbed the Mississippi River, a major confluent, the Illinois River, has been degraded
by a gradual increase in sediment input and sediment resuspension. From 1958 to 1961 formerly productive backwaters and lakes
along a 320-km reach of the Illinois River changed from clear, vegetated areas to turbid, barren basins. The change to a system
largely controlled by abiotic factors was rapid and the degraded condition persists.
Traditional approaches to experimental design are poorly suited for detecting control mechanisms and for determining the critical
thresholds in large river-floodplains. Large river-floodplain systems cannot be manipulated or sampled as easily as small
streams, and greater use should be made of man-made or natural disturbances and environmental restoration as opportunistic
experiments to measure thresholds and monitor the recovery process.
Coauthors are listed in alphabetical order. |
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Keywords: | Disturbance Recovery River Ecosystem Mississippi River Illinois River |
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