29 Sep 2023

A Regional Guidebook for Applying the Hydrogeomorphic Approach to Assessing Wetland Functions of Forested Riverine Wetlands in Alluvial Valleys of the Piedmont Region of the United States

Abstract: The Hydrogeomorphic (HGM) approach is used for developing and applying models for the site-specific assessment of wetland functions. It was initially designed for use in the context of the Clean Water Act Section 404 Regulatory Program permit review process to analyze project alternatives, minimize impacts, assess unavoidable impacts, determine mitigation requirements, and monitor the success of compensatory mitigation. However, a variety of other potential uses have been identified, including the design of wetland restoration projects, projecting ecological outcomes, developing success criteria and performance standards, and adaptive monitoring and management of wetlands. This guidebook provides an overview of the HGM approach including classification and characterization of the principal alluvial riverine wetlands identified in the Piedmont physiography. Eight potential subclasses of Piedmont wetlands, including Headwater, Low- and Mid-gradient Riverine, Floodplain Depression, Footslope Seeps, Flats, Precipitation Depressions, and Fringe wetlands were recognized. However, the occurrence of Flats, Precipitation Depressions, and Fringe wetlands in the Piedmont, are uncommon and not generally associated with alluvial riverine systems which is the subject of this Guidebook. Detailed HGM assessment models and protocols are presented for the five most common Piedmont riverine subclasses: Headwater, Low- and Mid-gradient Riverine, Floodplain Depression, and Footslope Seep. For each wetland subclass, the guidebook presents (a) the rationale used to select the wetland functions considered in the assessment process, (b) the rationale used to select assessment models, and (c) the functional index calibration curves developed from reference wetlands used in the assessment models. The guidebook outlines an assessment protocol for using the model variables and functional indices to assess each wetland subclass. The appendices provide field data collection forms. In addition, an automated spreadsheet model is provided to make calculations.