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| n addition to flood-related problems, a variety of other earth-sciencerelated issues also affect land-, mineral-, and water-use decisions throughout the Missouri River basin. To identify and study these issues, the U.S. Geological Survey and the geological surveys of Colorado, lowa, Kansas, Minnesota, Missouri, Montana, Nebraska, North Dakota, South Dakota, and Wyoming have formed the Missouri River Basin Earth Resources Mapping Group. A major function of the group is to encourage collaboration among Federal, State, and local agencies on the collection, interpretation, and use of natural resource information within the basin. This multidisciplinary effort focuses on interactions between human activities and the natural systems of the region. The group has identified four regionally important categories of human activity that relate closely to a variety of earth-science issues: urbanization, agriculture, mineral or energy extraction, and human-induced changes to natural systems. Issues associated with urbanization include flooding, ground failure, shrinking and swelling soils, subsidence, earthquakes, and extraction of construction materials. Issues related to agricultural activities include ground- and surface-water consumption, land and water contamination from agricultural chemicals, depletion of soil nutrients, salinization, wetland loss, and accelerated soil erosion, sedimentation, and runoff from cultivated or overgrazed lands. Issues related to mineral and energy extraction include mine drainage, leaching of waste materials, land subsidence, and accelerated erosion and sedimentation from disturbed lands. Issues associated with changes to natural systems include accelerated erosion and sedimentation and the resulting impacts on wetlands and associated habitats. These issues, although resulting from activities within the basin, may have far-reaching effects outside the basin as well.
W.H. Langer is USGS coordinator of the Missouri River Basin Resources Mapping Group and an expert in regional geologic mapping and analysis.
based on a specific regional and national need. The team's experience results in lessons not only for science but also for science management and for the interaction between science and the decisionmaking process. It is advancing science both by research in floodplain dynamics and by applied research in floodplain and upland management. The team is investigating and testing various data sets as well as developing new ones according to need. It is a model for improving our understanding of data and information integration, management, and dissemination and is a proving ground for data networking. Both Government and private organizations are already basing restoration and pilot field study decisions on SAST data. The SAST has proven to be an excellent example of a temporary, high-performance interagency organization, the results of which will be valuable for years to come.
John Kelmelis is the Director of the Scientific Assessment and Strategy Team and was a member of the IFMRC.
Impact of the 1993 Floods in the Upper Mississippi River Basin
he 1993 floods in the upper Mississippi
River basin were without precedent in modern North America. As measured by precipitation amounts, river levels, flood duration and extent, and economic losses, they surpassed any previous flood in the history of the United States. Seasonal rainfall records were broken in 9 Midwestern States, 95 National Weather Service forecast points experienced record flooding, and 45 U.S. Geological Survey (USGS) gaging stations recorded peak discharges exceeding the 100year-flood value; many of these stations remained above flood stage for weeks.
These floods clearly showed that people and property throughout the Midwest remain at risk to the hazard of riverine flooding; as the Nation's population continues to increase, this risk is also increasing. Average annual riverine flood damages have exceeded $3 billion The upper Mississippi River basin is one of several ecosystems targeted by the Department of the Interior for intensive investigations and coordination activity. The U.S. Geological Survey and other agencies are contributing scientific expertise as one of the cornerstones in resource decisionmaking. Enhanced scientific understanding will improve ecosystem management and help resource-management agencies generate solutions that successfully resolve environmental problems.
Among the principal–and not surprising– findings of postflood analysis is that flooding cannot be fully controlled; therefore, the risk of flooding cannot be wholly avoided. Moreover, the costs of flooding have become so high that they cannot be fairly or adequately shared without an aggressive program of damage avoidance and mitigation. Thus, the conceptual foundation of floodplain management needs to be redefined. Informed floodplain management requires a new scientific focus, one that considers not only where and how often but also precisely how it will flood. How energetically? With what concentration of force? With what potential for structural damage or for extensive scour and deposition and where? Detailed geologic, geomorphic, and hydrologic assessments of floodplains are required to fully answer these questions and thus to provide timely and relevant information to support a coordinated program of floodplain management by Federal, tribal, State, and local government agencies. To learn the answers to these vital questions, USGS scientists are studying the lower Missouri River floodplain between Kansas City and St. Louis, Mo. Mapping and analytical activities include: • Mapping the geomorphology and surficial geology of the floodplain.
Landsat Thematic Mapper satellite image of the area near Glasgow, Mo., showing flood conditions on August 1, 1993, 3 days after the main flood crest in this area. River flow at this time was approximately 60 to 80 percent of peak flood flow. Even at this time, flood waters (shown in black) covered the entire floodplain from bluff to bluff in most areas. One small cloud obscures a small area of the floodplain in the left-central part of this Image.
• Compiling an archive of historical channel and floodplain change.
• Analyzing regional patterns of floodinduced erosion and sedimentation.
• Developing conceptual models for predicting levee-failure risks and impacts.
Informed floodplain management requires a new scientific focus, one that considers not only where and how often but also precisely how it will flood.
These activities are being conducted as part of the Scientific Assessment and Strategy Team effort collaboratively with researchers from several Federal agencies, including the National Biological Service, the U.S. Army Corps of Engineers, the Soil Conservation Service, and the National Aeronautics and Space Administration. The results will be communicated to policymakers and other interested groups through a coordinated series of printed maps and digital map products.
Different Rivers--Different Reaches=Different Flood Impacts
omparison of the effects of the 1993
floods on the upper Mississippi and lower Missouri Rivers shows that river reaches in similar physiographic regions may respond very differently during floods. Although these rivers and their larger tributaries share a number of common features, significant differences in river discharge and slope, floodplain width, and sediment load strongly affect flood response. For example, the floodplain of the lower Missouri River is, on average, about twice as steep and half as wide as the floodplain of the middle Mississippi. Consequently, impacts of the 1993 flood were very different along these two
rivers. Levee breaches along the lower Missouri River commonly resulted in high-velocity flows across its relatively narrow and relatively steep floodplain. These high-energy flows caused extensive deep scour and thick sand deposition across prime agricultural bottomlands. In contrast, levee breaches along the middle Mississippi River produced fewer areas of intense erosion and sedimentation, and impacts were largely limited to passive inundation of large bottomland tracts. Where the floodplain is narrow, the meandering river channel partitions the floodplain surface into distinct segments, each almost completely bounded by the river channel and the valley sides. These areas, locally termed “bottoms,” occur as two distinct morphologic types: the loop bottom, a relatively small, roughly symmetrical bottomland area bounded by a single, continuously curving meander loop, and the long bottom, a larger, roughly rectilinear segment bounded by a straight channel reach that typically runs along the valley wall. Of the two, the loop bottom is much more prone to severe and extensive flood-induced scour and deposition. An important characteristic of the Missouri River, like all meandering rivers, is the migration of its meandering channel and the associated reworking of its floodplain surface. Areas reworked during the past two centuries are underlain primarily by channel sand and gravel that are capped by only minor amounts of overbank silt and clay. Thus, they provide relatively poor foundations for agricultural levees, and levees located in these areas are more prone to catastrophic failure. On average, areas reworked during this time form approximately one-third of the floodplain; however, most loop bottoms have been almost completely reworked. The downstream gradients of most loop bottoms are significantly steeper than the average downstream gradient of the floodplain. These steeper gradients are produced when fast-moving water overflows the upstream margins of meander loops, building thick sand deposits. In contrast, the downstream positions of loop bottoms are usually flooded much more passively. Where the overflows have been concentrated by levee failures, the more energetic of them also cause extensive scour; if they are channeled along existing depressions, these sedimentladen high-energy flows can transport large quantities of coarse sand and gravel more than 1.6 kilometers from the river channel. In contrast, the downstream portions of loop
Annotated Landsat Thematic Mapper satellite image summarizing the principal impacts of the 1993 floods in the area near Glasgow, Mo. Areas buried by significant thicknesses of sand are shown in white, the main stem channel of the Missouri River is shown in gray, and other areas of flooding are shown in black.
bottoms are usually flooded much more passively. Where they are unprotected by levees, these downstream areas (being lower than the average level of the floodplain) are flooded first by backwater overflow. This relatively passive backwater flooding does not transport bedload efficiently; thus, deposits from this flooding are generally thin and widely dispersed.
Floodplain Flow-Energy Zones
he floodplains of the lower Missouri and
middle Mississippi Rivers can be divided into at least four flow-energy zones: (1) the active river channel (artificially constrained into a relatively deep, narrow channel by the engineered structures of the present navigation channel system), (2) the active highenergy floodplain (the land surface adjacent to the existing river channel constructed primarily by the river in both its natural and structurally controlled regimes), (3) the lowenergy floodplain (one or more river terraces marginal to the active floodplain area that represent older, higher levels of the river valley), and (4) the highest river terraces (not inundated by the 1993 floods and at least 10,000 years old). Where the floodplain is narrow, the active channel and high-energy zones occupy most of the floodplain surface; where the floodplain is wide, the low-energy zones typically predominate. These natural floodplain zones were impacted differently by the 1993 floods, when the high-energy floodplain was actively flooded to depths of 3 to 6 meters, and extensive areas of deep scour and of thicksand deposition were created by local concentration of the rapidly flowing flood water. All other areas were much less significantly affected.
Levee-Induced Scour and Sedimentation Effects
E. from the 1993 flooding suggests that the present system of levee protection along the lower Missouri River is not working. During the 1993 flood, levee