Predicting Riparian Wetland Community Transitions using Hydraulic Metric Thresholds Derived from Dynamic 2D Modeling

Abstract

Agricultural expansion and flood mitigation activities have had unintentional, negative impacts on threatened, critical, and at-risk riparian wetland ecosystems in the Midwestern United States. Resulting changes in land use, stream channelization and levee construction have amplified bank erosion and channel degradation and have reduced floodplain connectivity. Projected increases in both drought and flood severity for the region further complicate hydrologic and hydraulic interactions between river channels and wetlands and create challenges for predicting ecosystem response. Hydraulic changes occurred gradually and abruptly in the form of changing water tables, log jams, and avulsions. In order for wetland managers to appropriately protect critical wetlands or adapt management to forecasted changes, a deeper understanding of the effect of altered hydraulics on wetland communities is needed. Important work has been completed pertaining to the relation of hydraulic traits and individual aquatic plant species and plant traits; however, large scale dynamic hydraulic changes and effects on wetland ecosystems are still poorly understood. Prior limitations to studying this problem included insufficient modeling tools for processing two dimensional systems and complexity of the system across multiple wetland species. Recent advances in modeling technology enabled study of two-dimensional (2D) hydraulics across entire floodplains and varying ecosystems. This project investigated the effect of flood inundation dynamics on wetland ecosystems. The analysis simulated two-dimensional channel and floodplain hydraulics for a sub-basin of the Grand River in the central United States (U.S.). U.S. Geological Survey (USGS) flow records for Locust Creek at Linneus, Missouri (MO); Grand River near Fountain Grove; and Grand River near Sumner, in addition to U.S. Army Corps of Engineers (USACE) Hydrologic Engineering Center (HEC) Hydrologic Modeling Software (HMS) generated flows were used to simulate a continuous period of record for Pershing State Park over a 10-year time period. Records of log jam formation and removal obtained from the Missouri Department of Natural Resources (MoDNR) were used to estimate changes to channel capacity and split flows. Hydraulic parameters from the HEC River Analysis System (RAS) 2D unsteady flow model were extracted annually and used to determine changes in wetland communities. Changes in the hydraulic parameters including inundation duration and depth were compared and tested as predictive variables for changes in wetland community boundaries. This project used overall wetland community metrics instead of indicator species for evaluation. Predicted transitions in wetland communities based on the hydraulic model simulations were compared to 2011 MoDNR and 2019 USACE ecosystem observational surveys to determine if hydraulic habitat suitability metrics predicted spatial changes in wetland communities. The analysis identified that inundation duration and inundation depth habitat metrics were overall predictive of wetland community transitions for bottomland hardwood forests and wet prairie. Inundation duration less than the specified threshold was a better metric than inundation depth for predicting both bottomland hardwood forest and wet prairie community gains or spatial expansion. Conversely, inundation depth exceeding the specified threshold was a better metric than inundation duration for predicting bottomland hardwood forest loss. Finally, wet prairie inundation depth thresholds could be used to predict both community gains and community losses depending on the selection of the habitat threshold magnitude. Overall, this thesis demonstrated that there was no singular hydraulic habitat metric that captured both community gains and losses. This improved understanding of the influence of large-scale hydraulics on wetland trajectories will help resource managers adapt to the changing hydraulic conditions and inform decisions of how to allocate monitoring, sampling, and rehabilitation resources to manage wetlands more efficiently and effectively.