Effects of Hydrologic Modifications on Flooding in Bottomland Hardwoods

Abstract

Complex fluvial processes influence floodplains. River modifications in the 1930s have affected hydrogeomorphic processes influencing the lower White River in southeastern Arkansas. The overall objective of this study was to better understand the hydrologic and geomorphic influence on the floodplain forest. We used the HEC-RAS model to quantify hydrologic relationships within the floodplain before and after 1930s river modifications. The model can replicate flooding within 3-5 m. Despite river modifications, HEC-RAS modeling showed headwater floods influenced the upper reach of the floodplain while backwater floods from the Mississippi River influenced the lower reach of the floodplain. Post-1930s incision that occurred from the confluence to the middle reach of the lower floodplain reduced the flooding extent primarily in frequent (< 5-year return interval) headwater floods. In contrast, incision only reduced flooding extent in the smallest (1-year return interval) backwater floods, and larger backwater events were largely unaffected. Modeled flooding regimes for PNV classes were more distinguishable among floodplain reaches than among PNV classes. The upper reach in the floodplain flooded more often from headwater floods, but the lower reach in the floodplain flooded deeper by backwater floods. Post-1930s incision reduced flooding depth and flooding extent the most in the riverine backwater upper zone , riverine backwater lower zone, and riverine overbank natural levee classes. The largest reductions in flooding depth and extent within these classes occurred during the more frequent floods, which are most important for ecological processes