Hydrogeomorphic Approach

Assessing Ecosystem Functionality

Published Jan. 18, 2013
The HGM Approach assesses functionality of a wetlands ecosystem by measuring interactions between its structural components and the surrounding landscape.

The HGM Approach assesses functionality of a wetlands ecosystem by measuring interactions between its structural components and the surrounding landscape.

Assessing Ecosystem Functionality

Wetlands are a precious environmental resource in the United States. These ecosystems improve water quality, recharge water supplies, reduce flood risks, and provide habitats for fish and other wildlife. To maintain valuable wetlands, researchers need a quick and efficient way to determine how well an ecosystem’s soil, plant life, animal life, and other structural components interact with their surrounding environment.

Flexible Approach for Many Applications

Developed by ERDC’s Environmental Laboratory in Vicksburg, Miss., the Hydrogeomorphic (HGM) Approach assesses the functions of a wetlands ecosystem by analyzing the physical, chemical, and biological interactions of the ecosystem’s structural components with the surrounding landscape. Functions measured by the HGM Approach include:

  • Hydrologic functions, such as storage of floodwater and protection of shorelines
  • Biogeochemical functions, such as improvement of water quality
  • Biological functions, such as providing habitat for aquatic and terrestrial flora and fauna

The HGM Approach can be used to assess a variety of wetland types over broad geographic areas, such as wet pine flats on mineral soils, or for assessment of streams and other aquatic ecosystems.

Easy-to-Use Method for Rapid, Reliable Assessments

The HGM Approach allows for quick assessment of ecosystem functions based on the deviation of measured or estimated variables from reference wetlands of the same type. The HGM Approach uses a classification method based on hydrogeomorphic position and hydrologic characteristics to group ecosystems into specific subclasses of seven HGM classes of wetlands. This method allows researchers to assess current wetland conditions, mitigation ratios, post-project impacts, and restoration success. Because the HGM Approach requires no specialized training or equipment, it can be easily applied at nearly any time of year within cost, manpower, and time constraints.

Regional HGM Guidebooks for Specialized Guidance

The HGM Approach can be implemented at local or regional levels, and regional guidebooks provide the user with information and data forms necessary to complete an assessment for an ecosystem subclass(s) and geographic region. Regional Guidebooks contain models to assess the functions of a particular wetland type in a defined geographic area. The output of each model is an index on a scale of 0 to 1 that reflects the functionality of the target wetland in relation to similar sites in the region, as well as the human impact on the wetland’s condition.


The Mississippi Valley Division Army Corps of Engineers has used the 2004 Arkansas Delta HGM guidebook to plan applications in all three of its districts. Where applications have involved large or complex projects, the Districts have requested ERDC assistance to gather data, define project alternatives, and to calculate project impact and mitigation obligations associated with each alternative.

ERDC supported the HGM analysis for the Bayou Meto Project (Memphis District), the White River Cutoff Project (Little Rock), and the Southeast Arkansas Project (Vicksburg). ERDC also applied the Yazoo Basin HGM guidebook to support Vicksburg District in planning analyses for iterations of the Yazoo Pumps Project (Vicksburg).

The Corps' planning model certification policy was not yet in place when those projects were completed. In 2009, Memphis District asked ERDC to assist with application of HGM to a large planning effort, the St. Johns-New Madrid Project (SJNM). During this project ERDC submitted a revised version of the Arkansas Delta HGM Guidebook for certification, which extends the applicability of the guidebook to the SJNM project area. The National Ecosystem Planning Center of Expertise (ECO-PCX) subsequently determined that "...the current version of the model is technically sound and is appropriate for its intended purpose."

Since then, ERDC has been working to unify the Arkansas Delta and Yazoo Basin guidebooks and widen their applicability to the entire Lower Mississippi Alluvial Valley (MAV). Upon certification of that updated and unified guidebook, it will be available for planning applications throughout the MAV region.

Distribution and Status

Reports and calculators for determining the Functional Capacity Index (FCI) can be downloaded - see RELATED LINK(s).


ERDCinfo@usace.army.mil, 601-634-3622
Updated 25 August 2020

Intro to the OHWM Manual

Video by Jared Eastman
Introduction to the Interim Draft of the National Ordinary High Water Mark (OHWM) Manual
U.S. Army Corps of Engineers, Engineer Research and Development Center
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Introduction to the Interim Draft of the National Ordinary High Water Mark (OHWM) Manual for Rivers and Streams.