PREFERENTIAL FLOW PATHS IN OXBOW WETLANDS: OXIDIZING SATURATED SOILS AND GROUNDWATER RECHARGE THROUGH CLAY

Recent studies in Sky Lake, an oxbow lake-wetland system in northwest Mississippi, challenge two commonly held assumptions. The first assumption is that the fine-grained sediments that typically infill oxbow lakes create a barrier to vertical flow, effectively eliminating the lakes as a source of groundwater recharge. However, many of the oxbow lakes in the region have perimeter wetlands that host forests of cypress and tupelo trees with extensive root systems. Decaying roots and fallen limbs have the potential to create preferential pathways to the underlying sands. Preliminary water-level results from a well in the center of the meander loop suggest the local aquifer is receiving recharge directly from the surrounding oxbow. A second assumption is that saturation of wetland sediment leads to reducing redox potential that is independent of subsequent changes in water depth. At Sky Lake, however, long-term measurement of soil redox potential at depths of 0.3 and 0.6 m have found isolated zones where the redox potential consistently rises to oxidizing conditions when wetland water depths exceed approximately 1 m. The response is attributed to the downward movement of oxygenated surface water through preferential flow pathways.