THE USE OF ARCHIVAL DATA, GEOSPATIAL DATABASES, AND RETRO-MODELING TO ASSESS MAN-MADE CHANGES TO THE MISSISSIPPI RIVER SYSTEM

During the past 100-200 years, much of the Mississippi River System (MRS) has been transformed, primarily to accommodate river navigation and to control floods. Large-scale structural measures have included emplacement of levees, wing dikes, bendway weirs, bridges, navigational dams, artificial meander cutoffs, and constriction of the channel. The long history of modification of the MRS has resulted in a wealth of historical hydrological and geospatial data which consists of maps, structure-history databases, and other quantitative measurements that stretch back 100 to >200 years. In some cases these data predate or coincide with the earliest significant river development, creating the potential to quantitatively document detailed historical baseline conditions from which change can be assessed.

We have attempted to assemble all data sources for the navigable Mississippi River (St. Paul, MN to the Gulf of Mexico), the Lower Missouri River (Sioux City, IA to the Missouri-Mississippi confluence), and the Illinois River (Lockport, IL to the Mississippi-Illinois confluence; >4,500 total km of river) To date we have digitized 3,086 hydrographic surveys maps, 1,611 other maps, and ~ 6 million stage and 1 million discharge observations. Forty of the total 79 map sets have been georeferenced, standardized to a uniform coordinate system and datum, and stored in a GIS. We also have digitized several river and engineering parameters including: bathymetry, land cover, bridges, navigation dams, cutoffs, wing dikes and levees.

One approach by which we use these data to assess change is “retro-modeling”. Retro-modeling involves the use of archival hydrologic and geospatial data in hydraulic models, such as HEC-RAS, to assess historical conditions. Comparison of modern conditions with historical reference conditions provides an assessment of change. Results of such modeling have revealed large-scale reductions in flood conveyance along modeled reaches of the Mississippi River during the 20th century. We have attributed the loss of flood conveyance to increased roughness of the floodway coupled with reduction in channel and floodplain area due to wing-dike and levee construction.