Efficacy of the hydrogeomorphic model (HGM): A case study from western Tennessee

Hydrogeomorphic models (HGM) for wetland classification and functional assessment have been developed for several regions in the United States. However, validation of models is lacking, even though models are already in use and the general idea is that each is a working model, developing over time with new ideas and information. We examined the HGM developed for western Tennessee using novel data collected from six stream systems, two channelized, two channelized and leveed, and two unchannelized. The western Tennessee HGM has the ability to differentiate altered and unaltered rivers based mainly on changes to hydrologic floodplain functions, although functional capacity index values were validated from independent measures. Effects of channelization were confounded in western Tennessee by other disturbances, channel recovery processes, and variability due to levee construction. Levees render the greatest change to floodplains as they disconnect the channel and floodplain system, decreasing water tables and possible exchanges. One of the main factors controlling productivity along channels without levees was sedimentation. We recommend that estimated rates of sedimentation be incorporated into an updated version of the HGM. Variability of human alterations and natural floodplain heterogeneity suggest landscape factors (e.g., beta diversity, degree of ponding) and use of gradient indices may result in greater resolution for determining functional capacity of low gradient, riverine systems.