Human impacts on the major rivers of Central North America have radically changed the characteristics of the rivers and their relationships to the floodplains. For much of the lower Missouri River , navigation enhancement structures have narrowed the channel to a small fraction of its natural width and complexity (NRC-CMRES, 2002). In addition, agricultural and industrial levees constructed by private interests and government agencies have cut the rivers off from their floodplains. The complex and dynamic large-river systems of Central North America no longer exist.
Flood Control?
While human modifications to rivers have allowed for commercial navigation and commercial development of the floodplains, these modifications are not without a price. Scientists have been suggesting since the early 1970’s that structural modifications to rivers (levees, wing dikes, dams, etc.) are impacting flood frequency and severity (Belt, 1975).
More recently, Dr. Nicholas Pinter has used the “specific-gage technique” to perform an analysis of flood stage trends on the Mississippi and Missouri Rivers . Pinter concluded that flood stages on these two rivers have increased over time due to the construction of dikes and levees that constrict the river channel and reduce its capacity to convey water (Pinter et al., 2000).
A wing dam on the north bank of the Missouri River .
Other studies have shown that river stages have increased between 6 and 13 feet for similar discharges at several stations on the Mississippi and Lower Missouri Rivers . No such increase was observed on the far upper Missouri and the Meramec Rivers , neither of which have been subjected to extensive engineering projects (Criss and Shock, 2001). This new model of flood dynamics indicates that the stages recorded during the 1993 flood on the Missouri River at Booneville , Missouri should recur every 15-20 years, instead of the 100- to 500-year interval predicted by the older models (Pinter and Heine, 2002). These indications have been further reinforced by a study showing flood stages at constant discharge have risen between 4 and 9 feet at seven long-term gauging stations on the lower Missouri River (Criss 2002).
The changes in river dynamics will continue to have profound impacts on those who live near the river. For example, the large levees currently under construction in St. Louis and those planned in the Kansas City and Jefferson City areas may not give the protection they are purported to provide. In fact, they will likely increase the flood damage potential for surrounding areas and facilitate at-risk development behind the levees. Nevertheless, the Corps continues to facilitate and permit these projects, which are usually under its jurisdiction, in apparent disregard of the long-term impacts on flood heights.
A dredger moored near the confluence of the Missouri and Mississippi Rivers .
River and Floodplain Ecology
Flooding is a natural and necessary component of the river-floodplain ecosystem. Periodic floods allow the various habitats associated with a river basin – including the river channel, backwaters, floodplain lakes, forests and wetlands – to exchange nutrients and organisms. The exchange supports biological productivity and diversity. The lower Missouri River is one of very few remaining stretches of large river-floodplain systems in the world that retain flood pulses and floodplains ( Sparks , 1995).
Levees and navigation or channelization structures disrupt the flood pulse and river floodplain connectivity. This disruption reduces fisheries productivity and increases nutrient loading in the river due to loss of riparian vegetation. Increased nutrient runoff in the Mississippi River basin is the apparent cause of hypoxia in the Gulf of Mexico that results in an ever growing dead zone.
Flood control dams reduce the sediment load in the water released from the dam. This water is able to pick up sediment, causing degradation (deepening) of the river channel in the reach below the dam. Sparks (1995) notes that this deepening occurs at least 346 km downstream of the Gavins Point dam on the Missouri River . This results in a lower water level in the river relative to the floodplain and a lower water table. Floodplain wetlands are lost as they dry out due to loss of connection to groundwater.
Bank armoring and stabilization structures prevent the
Missouri River from meandering and creating new habitat.
Bank stabilization and channelization structures have had another unanticipated effect on the floodplain ecosystem. Cottonwood reproduction has effectively ceased in the large river floodplains of central North America . Cottonwood does not reproduce in undisturbed forest. An actively meandering river that creates new land is required for new cottonwood colonies to become established. It is a disturbance dependent species that declines in abundance in stable floodplain areas (NRC-CMRES, 2002). Since river meandering has been essentially stopped by engineering structures, cottonwood forests are disappearing and new groves are not appearing.
Clearly Man’s flood control and river engineering activities have had a tremendous impact on river dynamics and ecology. MCE is working to mitigate and restore natural function to Missouri ’s rivers and floodplains. Sound science is used to back aggressive advocacy and litigation for the rivers.
References:
Belt, C.B., (1975). The 1973 flood and man’s constriction of the Mississippi River . Science, Vol. 189, p. 681-684.
Criss, R.E. and Shock, E.L., (2001). Flood enhancement through flood control. Geology, Vol. 29 No. 10; p. 875-878.
Criss, R.E., (2002). Rising flood stages on the lower Missouri River . ftp://ftp.ewgateway.org/library/wrc/rising_flood_stages.pdf
National Research Council – Committee on Missouri River Ecosystem Science. (2002). The Missouri River Ecosystem: Exploring the Prospects for Recovery. Washington , D.C. : National Academy Press.
Pinter, N., Thomas, R., and Wlosinski, J.H., (2000). Regional impacts of levee construction and channelization, middle Mississippi River , USA . In Marsalek, J., et al., eds., Flood issues in contemporary water management: Kluwer Academic Publishers, Boston , Massachusettes., p. 351-361.
Pinter, N. and Heine, R. (2002). Hydrologic History of the Lower Missouri River . http://216.114.78.114/webcenter/sites/mce/images/Pinter%202002.pdf
Sparks, R.E. (1995). Need for ecosystem management of large rivers and their floodplains. Bioscience, Vol. 45 No. 3; p. 168-182. |
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