GEOMORPHOLOGY AND HYDROGEOLOGY OF MISSISSIPPI RIVER ISLANDS AND POTENTIAL TO AFFECT NITROGEN CYCLING NEAR MEMPHIS, TN, USA

The Lower Mississippi River (LMR) has numerous mid-channel bars and vegetated islands that could play a significant role in nitrogen cycling, similar to that observed in riparian wetland environments. Analysis of historical river stages and aerial photos in the Shelby County reach reveal that intrariver land area to water area ratios range from 1:3 at low stage and 1:5 at high stage supporting the hypothesis that river bars and islands are of a significant size to influence downstream nitrogen transport.

A representative non-vegetated island, Densford Bar (DB), a vegetated island, Loosahatchie Bar (LB), and a river bank site at Robinson Crusoe Island (RCI) were chosen for stratigraphic and hydrologic analysis. Shallow wells were installed at several points on each island to assess stratigraphy and make hydrologic measurements. Soil samples were described in the field then later classified using sieve analysis. These analyses revealed that DB is mostly sand with minor lenses of fine material. LB displayed alternating layers of fine sands and silty clay representing accreted deposits from flood events. RCI consists of predominantly fine sediment and organic layers attributable to seasonal leaf litter and flooding events. Conceptual models of each site suggest DB behaves similar to a single, unconfined aquifer with potential perched zones, whereas LB may act as a series of confined to semi-confined aquifer units. Both islands are interpreted to be dominated by radial, lateral drainage as river stage decreases. Observations of island sedimentation processes indicate that the vegetated island is more susceptible to vertical accretion, whereas the non-vegetated island is more susceptible to lateral bar migration. Representative soil samples from each 0.3 m borehole increment were subjected to Loss on Ignition (LOI) analysis and select samples were analyzed for total carbon (TC) and total nitrogen (TN). The highest TC and TN values were observed in LB and RCI samples with the highest organic content, suggesting increased potential for nitrogen transformation and/or entrapment in those locations. These findings suggest that islands could potentially play a significant role in nitrogen cycling and the overall nitrogen budget of the LMR.