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Wetland Functional Assessments

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Wetland Assessments

Wetlands are habitats transitional between open water and terrestrial (dry) land, where water ponds or floods regularly or all the time.   Wetlands serve many functions.   Those functions vary depending on the type of wetland ecosystem and where the wetland is, but most wetlands have similar functions.   The most common type of wetland is found along and within streams, referred to as Riverine and Palustrine wetlands.   A typical riverine wetland along a stream has 14 functions, including:
  • Energy dissipation (slowing down the flood waters);
  • Surface & subsurface water storage & exchange (making water available to animals and underground aquifers);
  • Landscape hydrologic connections (connecting habitats together for wildlife and plants);
  • Sediment mobilization, storage, transport, & deposition (moving and storing sediments);
  • Cycling of elements & compounds (recycling on nutrients from leaves and such);
  • Removal of imported elements & compounds (moving sediment and leaves and such downstream);
  • Particulate detention (holding sand, silt, and clay in the floodplain);
  • Organic matter transport (moving organic matter around);
  • Plant community (providing habitat for plants that like or tolerate periodic or permanent flooding);
  • Detrital biomass (creating and storing decaying leaves and such);
  • Spatial structure of habitats (providing multiple places for wildlife and plants to live);
  • Interspersion & connectivity of habitats (providing a means of connecting different types of habitats together);
  • Distribution & abundance of vertebrate taxa (providing habitat and movement routes for animals); and
  • Distribution & abundance of invertebrate taxa (providing habitat and movement routes for insects and similar types of animals).
Assessing and understanding the functions of wetlands is sometimes required by regulatory agencies as part of their regulatory process.   For example, the U.S. Army Corps of Engineers must permit only the least environmentally damaging project alternative, focusing on impacts to wetlands, pursuant to Section 404(b)(1) of the Clean Water Act.   The use of hydrogeomorphic (HGM) methodology allows riverine wetland ecosystem functions to be characterized in a quantitative manner.   HGM assessments can be used in a systematic and objective approach to measure the relative changes in wetland functions over time resulting from natural processes or from human activity.

HGM offers an efficient and reference-based set of field techniques for the assessment of ecosystem functioning of waters/wetland ecosystems.   The HGM functional assessment protocol is based on scientifically defensible methods for rapid assessment of wetland ecosystem functions.   Specifically, with modest training and practice, HGM allows users to document the level of ecosystem functions through the assessment of field indicators of these functions.   The HGM method departs from other functional assessment approaches (e.g. Wetland Evaluation Technique [“WET”]) in that it is based upon three fundamental steps:
  • Recognition of differences among waters/wetlands (i.e. classification);
  • Identification of ecosystem functions performed by broadly circumscribed classes and subclasses of wetlands; and
  • Regionally developed reference systems.
Baseline conditions are established in an initial assessment at the start of a timeline or before a project begins.   Subsequent assessments are then carried out at specified time intervals or at particular stages of a project to determine if or how much wetland functions have changed.

HGM methodology can be used to estimate the changes in wetland functions expected to result from a proposed project.   After the values of wetland functions under baseline conditions are determined, expected changes in the values due to project features are used to calculate the estimated level of impacts resulting from project implementation.

For planning purposes, an analysis of expected project impacts can be used to modify the project design so that impacts are minimized.   Such an analysis also makes it possible to identify ways in which expected impacts can be mitigated in order to support project approvals.

HGM protocols are based on complex hydrogeomorphic models that describe critical stream ecosystem functions primarily in terms of the degree to which natural channel morphology and native plant communities have been maintained relative to reference standard (pristine) conditions.

There are three regional riverine wetland HGM models used in California coastal areas: South Coast Santa Barbara Streams HGM, Santa Margarita Watershed HGM, and Central Coast HGM.   DMEC has developed proprietary versions of each of these models to apply them efficiently to specific projects.

HGM assessments involve the measurement of numerous physical and biological features at locations along the stream reach in the project or study area as described by field protocols.   Other features in larger areas beyond and surrounding the project or study site are determined by Geographic Information System (GIS) analysis.

DMEC is both experienced with and experts in HGM wetland assessments.   HGM assessments conducted by DMEC staff include:
  • Mojave River/East Cronese Lake, Mojave River Watershed Mitigation Bank, Ojai, CA (DMEC 2014)
  • Santa Paula Creek, Santa Paula Creek Mitigation Bank, Ojai, CA (DMEC 2009)
  • Dry Canyon Creek, Mountains Restoration Trust, Calabasas, CA (DMEC 2006)
  • Live Oak Creek, Gramckow Property, Ojai, CA (DMEC 2006)
  • Calleguas Creek, California Coastal Conservancy, Camarillo, CA (DMEC 2004)
  • Las Flores Creek, Odyssey Program, Malibu, CA (DMEC 2001)
  • Conejo Creek tributary, Reinke development site, Thousand Oaks, CA (DMEC 2000)
  • Bridle Ridge Development, Santa Barbara County, CA (DMEC 1998)
  • Cohan Development, Thousand Oaks, CA (Magney while at ENSR 1997)
  • Los Osos Creek, Los Osos Sewer Project, San Luis Obispo County, CA (Magney while at Fugro West 1996)

See the “Projects” and “Reports” sections of this website for more details on some of these projects.

DMEC is presently developing a regional guidebook for Mojave Desert playa lake depressional wetlands.   The Mojave Desert contains numerous large playa lakes, some of which are considered under the jurisdiction of the Corps; however, there is no HGM guidebook for this type of wetland.   Therefore, to meet fill this void, DMEC is preparing a preliminary draft guidebook to cover these types of wetlands.
This page last updated 5 August 2014
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