The Mississippi River is one of the most intensively managed and altered river systems in the world. The alterations to the Mississippi have been largely made to meet navigation demands and mitigate floods. River training has been undertaken using rock structures, commonly referred to as river training structures (RTS), to modify the shape of the river to maintain the Congressional mandated navigation channel dimensions. In addition to maintaining the navigation channel, newer RTS such as chevrons, have been claimed to be designed as an improvement to the previously existing dikes. They are considered to be tools of improving riverine habitat by increasing physical habitat heterogeneity within the highly engineered and consequently uniform river channel. Thus, to evaluate the differences in physical habitat heterogeneity created by the two RTS; a dike and a chevron, this study models and compares the physical aquatic habitats created along the Middle Mississippi River near Grand Tower, Illinois. The hydraulic modelling software HEC-RAS has been used to develop a two-dimensional model of the study area containing the RTS using detailed 2 m- resolution topobathy digital elevation model (DEM), U.S Geologic Survey’s National Land Cover Database (NLCD), an existing one-dimensional model of the Mississippi and hydrologic data from several hydrologic monitoring stations for the years 2008-2016. Depth and velocity grids were extracted from the HEC-RAS model for three different discharge conditions; 0.5 mean annual flow (MAF) with 40% exceedance probability, MAF with 80% exceedance probability and 1.5 MAF with 15% exceedance probability were used to develop and categorize physical habitat distribution maps of the study area using ArcGIS. The modeled physical aquatic habitat patches were assessed at three buffer distances of 30 m, 90 m, and 150 m from the RTS. The area Simpson diversity and Jaccard similarity indices were calculated for the different discharge conditions and associated habitat mosaics. The distribution of physical habitat modeling revealed a variation in the pattern of habitat patches between the dike and chevron. For the chevron dike evaluated in this study, very-slow deep habitat patches are created in the inner portion of the chevron and slow deep patches around the exterior of the structure which extend both up and downstream of the structure. The dike created slow-deep habitat patches along the structure, very-slow deep patches on the riverbank edge and fast- deep patches on the river side edge. Evaluation of physical habitat patch diversity in relation to the distance from the RTS revealed the highest diversity index values were found within the first 30 m buffer and generally decrease with distance away from the structure. Comparison of the Jaccard index values in vicinity of the two evaluated RTS suggest the habitat patch diversity are similar for both structures at 0.5 MAF and 1.5 MAF flow conditions (index value ranging between 0.60-0.87). However, for the MAF flow conditions the Jaccard index suggests there is more physical habitat patch diversity in the vicinity of the chevron relative to the dike. The modeling results suggest both physical habitat patch richness and diversity declines with an increase in discharge. The decline in physical habitat patch richness and diversity with discharge conditions were greater for the dike relative to the chevron, thus while the chevron retains more types of habitat patches with increase in discharge the diversity indices are still higher for the dike. The modeling also suggests both RTS have created and maintain shallow water habitat (SHW) and overwintering habitat patches (OWH) for the flow conditions evaluated in this study. These habitat patches are utilized by fish species at various life stages. Larger area of OWH habitats; 30% of total area by dikes and 35% of total area by chevrons are created in comparison to SWH; 10% by dike and 7% by chevron. The modeling results show that both the dike and chevron evaluated in this study are associated with and likely maintain ecologically relevant habitats and substantially contribute to physical habitat diversity. If the physical characteristics of the RTS investigated here are similar to other dikes and chevrons along the Mississippi River, the results of this study supports the secondary management objective for these structures, increase in physical aquatic habitat patch diversity, is likely being achieved.
| Identifer | oai:union.ndltd.org:siu.edu/oai:opensiuc.lib.siu.edu:theses-3753 |
| Date | 01 September 2020 |
| Creators | Karki, Nimisha |
| Publisher | OpenSIUC |
| Source Sets | Southern Illinois University Carbondale |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Theses |
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