3.6 Wetland Delineation, Classification & Riparian Buffers

Wetland Delineation: The USACE Three-Parameter Framework

Under Section 404 of the Clean Water Act, the U.S. Army Corps of Engineers (USACE) and the Environmental Protection Agency (EPA) hold federal regulatory authority over "Waters of the United States" (WOTUS), including wetlands.

Federal Definition of Wetlands

"Those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions." (33 CFR § 328.3)

The Three Mandatory USACE Parameters

To legally delineate a jurisdictional wetland under the 1987 USACE Wetland Delineation Manual and its Regional Supplements, an area must simultaneously satisfy all three technical parameters:

USACE Three-Parameter Wetland Criteria:

             ┌─────────────────────────┐
             │ Hydrophytic Vegetation  │
             │  (>50% OBL, FACW, FAC)  │
             └────────────┬────────────┘
                          │
                          ▼
       ┌──────────────────┴──────────────────┐
       │                                     │
       ▼                                     ▼
┌───────────────┐                     ┌───────────────┐
│ Hydric Soils  │◄───────────────────►│    Wetland    │
│ (Gley/Chroma≤2│                     │   Hydrology   │
│ /Redoximorphic│                     │ (14 days sat. │
│  /Histosols)  │                     │ upper 12 in.) │
└───────────────┘                     └───────────────┘
              ALL THREE MUST CO-OCCUR!

1. Hydrophytic Vegetation

Plants adapted to grow in anaerobic (oxygen-deficient), waterlogged soils. Plant species are classified by the National Wetland Plant List (NWPL) into five wetland indicator categories:

• OBL (Obligate Wetland): Almost always occur in wetlands (>99% probability under natural conditions, e.g., Typha latifolia, Taxodium distichum).
• FACW (Facultative Wetland): Usually occur in wetlands (67% to 99% probability, e.g., Acer rubrum, Alnus incana).
• FAC (Facultative): Equally likely to occur in wetlands or non-wetlands (33% to 66% probability, e.g., Liquidambar styraciflua, Solidago rugosa).
• FACU (Facultative Upland): Occasionally occur in wetlands (1% to 33% probability, usually in uplands, e.g., Liriodendron tulipifera, Quercus alba).
• UPL (Obligate Upland): Rarely occur in wetlands (<1% probability under natural conditions, e.g., Pinus virginiana, Cornus florida).

The 50/20 Dominance Test: A plant community passes the hydrophytic vegetation test if more than 50% of the dominant species across all vegetative strata (tree canopy, sapling/shrub, herbaceous, woody vine) have an indicator status of OBL, FACW, or FAC. Dominant species are selected using the "50/20 Rule" (species that exceed 20% relative dominance, or species that cumulatively make up 50% of the total cover).

2. Hydric Soils

Soils that formed under conditions of saturation, ponding, or flooding long enough during the growing season to develop anaerobic conditions in the upper horizons. Anaerobic conditions drive soil bacteria to reduce oxidized ferric iron ($Fe^{3+}$, reddish-brown) into soluble ferrous iron ($Fe^{2+}$, clear/gray). Diagnostic hydric soil field indicators include:

• Gleyed Matrix: A soil horizon displaying neutral gray, bluish-gray, or greenish-gray background colors resulting from complete iron reduction. Evaluated using the Munsell Soil Color Chart; a matrix with a chroma of 2 or less (and value of 4 or higher) indicates a hydric soil.
• Redoximorphic Features (Mottles): Concentrated spots or splotches of bright orange/red/yellow oxidized iron ($Fe^{3+}$) and black manganese ($Mn$) where oxygen periodically re-enters through root channels or seasonal drying.
• Histosols (Organic Soils): Thick accumulations of organic muck or peat (typically > 16 inches of organic matter) resulting from anaerobic conditions halting plant decomposition.
• Sulfidic Odor: Strong "rotten egg" odor produced by hydrogen sulfide gas ($H_2S$) generated by sulfate-reducing bacteria in permanently saturated, anaerobic soils.

3. Wetland Hydrology Indicators

Evidence that the site undergoes continuous inundation or soil saturation within the upper 12 inches of the soil profile for at least 14 consecutive days during the growing season.

• Primary Indicators (Any ONE confirms hydrology):
  • Surface water visible on site.
  • High water table observed in a test pit within 12 inches of the surface.
  • Soil saturation within the upper 12 inches.
  • Water marks on woody tree trunks or structures.
  • Drift deposits (lines of rafted debris, leaves, and twigs lodged against vegetation).
  • Sediment deposits coating plant leaves and rocks.
  • Algal mats or crusts dried on the soil surface.
  • Oxidized rhizospheres (bright orange rust halos surrounding living plant roots, proving oxygen leakage into anaerobic soil).
• Secondary Indicators (Requires at least TWO):
  • Surface soil cracks.
  • Drainage patterns in wetlands.
  • Moss trim lines on tree trunks.
  • Geomorphic position (concave low-lying depression).
  • FAC-neutral test (evaluating whether OBL and FACW dominants outnumber FACU and UPL dominants).

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The Cowardin Wetland Classification System (USFWS / NWI)

Developed by Lewis Cowardin in 1979 for the U.S. Fish and Wildlife Service, this system forms the classification engine for the National Wetlands Inventory (NWI) maps. It divides all aquatic resources into Five Ecological Systems:

1. Palustrine (P): Encompasses all nontidal freshwater wetlands dominated by trees, shrubs, persistent emergents, and mosses/lichens, as well as small, shallow water bodies (< 20 acres, depth < 2 meters at low water). Over 90% of all inland wetlands are Palustrine.
   • Palustrine Forested (PFO): Swamps and bottomland hardwood forests.
   • Palustrine Scrub-Shrub (PSS): Shrub bogs, alder thickets, willow carrs.
   • Palustrine Emergent (PEM): Freshwater marshes, wet meadows, fens.
   • Palustrine Unconsolidated Bottom (PUB): Farm ponds, shallow retention basins.
2. Riverine (R): Freshwater wetlands and deepwater habitats contained within a natural or artificial channel (excluding wetlands dominated by trees, shrubs, or persistent emergents, which are classified as Palustrine even if situated in a floodplain).
3. Lacustrine (L): Deep, large water bodies situated in topographic depressions or dammed river channels, exceeding 20 acres (8 hectares) in size or with a deepest point exceeding 2 meters (6.6 ft) at low water, with < 30% emergent vegetative cover (e.g., lakes, deep reservoirs).
4. Estuarine (E): Tidal wetlands and deepwater habitats semi-enclosed by land, where ocean water is diluted by freshwater runoff from the land (e.g., salt marshes, brackish tidal marshes, mangrove swamps, coastal bays).
5. Marine (M): The open ocean over the continental shelf and its exposed, high-energy rocky or sandy coastlines with full oceanic salinity.

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Riparian Buffer Systems: Multi-Zone Widths & Functions

Riparian buffers are vegetated corridors situated adjacent to streams, rivers, wetlands, and lakes that protect water quality and aquatic ecology from adjacent agricultural or urban land uses.

The Standard Three-Zone Riparian Buffer Model

Developed by the USFS and widely adopted in municipal ordinances, the three-zone buffer system provides specialized ecological protections:

Three-Zone Riparian Buffer Cross-Section:

 ◄─── Zone 1 ───►◄────── Zone 2 ──────►◄─── Zone 3 ───►◄── Upland Land Use ──►
   Streamside         Middle Zone         Outer Zone       (Urban / Ag)
   (15 - 25')         (50 - 100')         (15 - 25')

    Mature Trees       Managed Forest      Dense Grass
      & Shrubs           Canopy            Filter Strip
       ┌───┐               ┌───┐
       │   │               │   │              \\\\\       [ Buildings / ]
     ┌─┴───┴─┐           ┌─┴───┴─┐            \\\\\       [   Parking   ]
~~~~~│ Root  │~~~~~~~~~~~│ Roots │~~~~~~~~~~~~│Roots│───────────────────────
Stream Bank   Bank Stab.  Denitrification  Sheet Flow    Runoff Source

Riparian Buffer Width Targets by Ecological Function

• Stream Bank Stabilization: 25 to 50 feet.
• Sediment & Particulate Phosphorus Trapping: 50 to 100 feet (must increase on slopes > 10%).
• Dissolved Nitrogen (Nitrate) Removal: 75 to 150 feet (requires shallow water table through organic forest soil to support anaerobic denitrification).
• Cold-Water Fishery Protection (Trout/Salmon): 100 to 150 feet of continuous forest canopy shade.
• Wildlife Movement & Interior Bird Nesting Corridors: 100 to 300+ feet of unfragmented habitat.

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LARE Exam Traps & Practical Scenarios

[!WARNING] Exam Trap 1: Assuming Cattails Equal a Legal Wetland Finding hydrophytic vegetation (like broadleaf cattail or red maple) does not prove a jurisdictional wetland. Landscape architects frequently encounter isolated depressions containing cattails that were created by stormwater discharge over compacted upland soils lacking hydric soil morphology. All three parameters (hydrophytic plants, hydric soils, wetland hydrology) must be present.

[!CAUTION] Exam Trap 2: Building in the Regulatory Floodway Candidates often confuse the 100-year floodplain (flood fringe) with the regulatory floodway. You can build in the flood fringe if the building is elevated to or above the BFE. You CANNOT build, fill, or grade in the regulatory floodway unless an engineered HEC-RAS hydraulic model proves a zero-rise (0.00 ft) impact on upstream flood elevations.

[!NOTE] Real-World Design Scenario: Waterfront Park Master Plan A city proposes a riverfront park with a kayak launch, festival lawn, and public pavilion. The FIRM shows the site is split between Zone AE (BFE = 112.0) and an interior regulatory floodway. The client demands locating the enclosed public pavilion inside the floodway for views. The landscape architect must advise the client that constructing an enclosed building in the floodway will fail federal and local permitting due to floodway encroachment restrictions. The solution: locate the permeable, open kayak launch and floodable turf lawn within the floodway (no rise in flood elevations), while shifting the enclosed pavilion upland into the flood fringe, setting its lowest finished floor elevation at 114.0 (BFE + 2.0 ft freeboard).