Jonathan W.F. Remo

New databases reveal 200 years of change on the Mississippi River system

Over the past two centuries, the Mississippi River system (MRS) has been dramatically altered to facilitate commercial navigation and provide flood control. Because of its long history of modification, rigorous measurements including maps, charts, surveys, and other data concerning the MRS are available from the past 200 years or longer. Comparison between historical reference conditions and modern conditions can document changes related to natural processes, human impacts, and river management practices and policies. To show these changes, we have assembled a hydrologic database of approximately 7 million measurements and a geospatial database consisting of 4878 map sheets dating back to 1765. The databases include nearly all available data sources for the navigable Mississippi River, the lower Missouri River, and the Illinois River (>4500 kilometers of waterways; see Figure 1, in the electronic supplement to this Eos issue; http://www.agu.org/eos_elec). We have digitized all 81 map sets and georeferenced 48 of those map sets, meaning that the maps have been referenced in physical space for use in a geographic information system. The purpose for constructing these databases is to centralize and standardize these data sources and to make them broadly available for research and management. In this article we describe the data, the databases, and their dissemination.

Climatic control of Mississippi River flood hazard amplified by river engineering

Over the past century, many of the world’s major rivers have been modified for the purposes of flood mitigation, power generation and commercial navigation. Engineering modifications to the Mississippi River system have altered the river’s sediment levels and channel morphology, but the influence of these modifications on flood hazard is debated. Detecting and attributing changes in river discharge is challenging because instrumental streamflow records are often too short to evaluate the range of natural hydrological variability before the establishment of flood mitigation infrastructure. Here we show that multi-decadal trends of flood hazard on the lower Mississippi River are strongly modulated by dynamical modes of climate variability, particularly the El Niño–Southern Oscillation and the Atlantic Multidecadal Oscillation, but that the artificial channelization (confinement to a straightened channel) has greatly amplified flood magnitudes over the past century. Our results, based on a multi-proxy reconstruction of flood frequency and magnitude spanning the past 500 years, reveal that the magnitude of the 100-year flood (a flood with a 1 per cent chance of being exceeded in any year) has increased by 20 per cent over those five centuries, with about 75 per cent of this increase attributed to river engineering. We conclude that the interaction of human alterations to the Mississippi River system with dynamical modes of climate variability has elevated the current flood hazard to levels that are unprecedented within the past five centuries.

Theoretical Analysis of Wing Dike Impact on River Flood Stages

The question of whether wing dikes (bank-perpendicular river training structures or groynes) cause higher flood levels has been debated in the United States for many years. Some researchers point to empirical data that show large stage increases which are associated with wing dike construction, whereas other researchers have suggested that such increases are contrary to engineering theory. In a recent report, the U.S. Government Accountability Office (USGAO) presented this question as a priority to be resolved by engineers and scientists. As a first step to better understand the connection between navigation structures and flood levels on the Middle Mississippi River (MMR), a simplified theoretical analysis is presented to test the assertion (made in the USGAO report and elsewhere) that such increases are contrary to hydraulic theory. This analysis predicts that wing dike construction may lead to water level lowering for in-bank flows and to water level increases for out-of-bank (flood) flows. This confirms that, in principle, wing dikes may have contributed to the observed flood water level trends in the MMR. More detailed follow-up studies are required to accurately quantify the impact of wing dikes on flood levels. DOI: 10.1061/(ASCE)HY.1943-7900.0000698.

The ups and downs of levees: GPS–based change detection, Middle Mississippi River, USA

Changes in levee-crest elevations were measured along 328 km of Mississippi River levees between St. Louis, Missouri, and Cairo, Illinois, in 1998 and 2007. We also compared 1998 and 2007 survey data with 50 yr fl ood elevation profi les to assess levels of levee protection. Levee heights, stability, and safety are nationwide concerns, especially since Hurricane Katrina and fl oodwall failures in New Orleans, Louisiana, in 2005. The majority of surveyed levees were stable at the centimeter to decimeter level during the 9 yr measurement interval. Change was measured in other areas, including increases of as much as 1.49 m and decreases of as much as 1.26 m between 1998 and 2007. The increases corresponded to local crevasse repairs, smallscale road maintenance, and larger levee-raising projects. Decreases in levee elevations are interpreted as small-scale surface erosion and larger compaction or subsidence of levee and/or foundation materials. Levee crests in 2007 were locally as much as 2.0 m below the 50 yr fl oodgrade elevations. Levee degradation reduces protection levels for fl oodplain residents, often without easily visible symptoms. Intentional levee raising without necessary studies, engineering, or permitting has a range of potential negative impacts including both a false sense of security and the potential for exporting fl ood risk to neighboring levee communities. Regional surveying of levee elevations and change over time can provide a preliminary tool for assessing levee conditions, stability, and compliance with levee regulations.

Tradeoffs of strategically reconnecting rivers to their floodplains: The case of the Lower Illinois River (USA)

During the latter half of the 19th Century and first half of the 20th Century, the Illinois River was heavily altered through leveeing off large portions of its floodplain, draining wetlands, and the construction of dams and river-training structures that facilitated navigation. As a result of these alterations, flood stages continue to rise, increasing flood risk and threatening to overtop levees along the La Grange Segment (LGS) of the Illinois River. Over the last two decades, more emphasis has been placed on reconnecting portions of floodplains to rivers in order to solve the long-term problem of rising flood heights attributed to continual heightening of levees to provide flood protection. Multiple studies have suggested that strategically reconnecting larger portions of the LGS could result in more sustainable floodplain management. However, the true costs and benefits of reconnecting the floodplain are not known. We use a novel hydrodynamic, geospatial, economic, and habitat suitability framework to assess the tradeoffs of strategically reconnecting the Illinois River to its floodplain in order to decrease flood risk, improve floodplain habitats, and limit the costs of reconnection. Costs include building-associated losses, lost agricultural profits, and levee removal and construction costs. Tested scenarios demonstrate that while flood heights and environmental benefits are maximized through the most aggressive levee setbacks and removals, these scenarios also have the highest costs. However, the tradeoff of implementing lower-cost scenarios is that there is less flood-height reduction and less floodplain habitat available. Several individual levee districts have high potential for reconnection based on limiting potential damages as well as providing floodplain habitat. To implement large-scale strategic floodplain reconnection, costs range from

The use of retro- and scenario-modeling to assess effects of 100+ years river of engineering and land-cover change on Middle and Lower Mississippi River flood stages

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Flood trends and river engineering on the Mississippi River system

4.3 billion. As such, payments for ecosystem services will likely be necessary to compensate landowners for decreased long-term agricultural production and building losses that result in flood-reduction benefits and increased floodplain habitat.