STORAGE OF WATER IN OXBOW LAKES FOR AQUIFER RECHARGE: HYDROLOGIC, GEOMORPHIC, AGRICULTURAL, AND ECOLOGICAL CONSIDERATIONS

Intensive agricultural irrigation has resulted in steady groundwater declines in the Mississippi River Valley alluvial aquifer, with subsequent attention given to methods of enhancing aquifer recharge. Surface water in oxbow lakes has historically been considered to be isolated from groundwater due to deposition of fine-grained bottom sediments. Recent studies in Sky Lake, an oxbow wetland-lake in the Delta region of Mississippi, have documented substantial recharge to the underlying aquifer when lake levels are high. Vertical transport appears to be highest where surface water spills out over sandy scroll-bar deposits inside the meander and in the forested wetlands where decomposing tree-remains create substantial sub-surface heterogeneity. These observations indicate that impounding water in some oxbow systems could be a cost-effective means of enhanced aquifer recharge. Multiple factors must be considered in creating and managing water storage in these systems. (1) Geomorphic: The ideal oxbow does not need to be deep, as recharge will occur primarily where water spills into perimeter wetlands or over scroll-bar deposits. A wide forested wetland or flooded scroll bar is preferable to a large open-water lake. (2) Agricultural: Scroll-bar deposits are often farmed during the summer months, limiting support for impoundment of water to the late fall and winter months. (3) Ecological: Oxbow lakes serve as permanent or temporal niches for many different organisms, some with competing needs. Many species of migratory birds benefit from mud-flats and shallow-water conditions in the early spring. Fish, on the other hand, struggle when rising temperature and shallow water results in oxygen depletion. With nearly flat bottoms, there is no deep-water refugia for fish to avoid anoxia (e.g. fish-kill event in Sky Lake in June, 2022). Cypress trees, a dominant canopy species in these systems, have a complex relationship with hydroperiod. Seeds require aeration to germinate. Saplings and adult trees can survive permanent inundation, but ongoing studies at Sky Lake document that the timing of inundation, depth of water, and elevation in the wetland contribute to variable responses to changes in water depth. Taken together, seasonal impoundment of water in winter is likely to be preferred over permanent high water.