Guide to National Wetlands Inventory Types and Functions

The Montana Wetland and Riparian Mapping Center was established in 2007 as the result of an EPA pilot grant. We follow the National Wetland Inventory (NWI) classification system to map wetlands and riparian areas.

In order to relate the NWI types with vegetation, management, and ecological functions, We examined wetlands in the Gallatin Valley and reference literature to link the common NWI types to National Vegetation Classification System types, other vegetation types (Hansen et al. 1995), and Hydrogeomorphic (HGM) wetland functions.

Since the HGM approach provides a mechanism to link wetland type with wetland function, our wetland mapping includes the HGM modifier. The addition of the HGM code will improve assessment and mitigation of wetlands in Montana. See the Gallatin Change Detection Report for more information.

In the NWI classification, most Montana wetlands are in the Palustrine System which includes any wetlands not within a stream or river channel (Riverine System) or within bodies of water > 20 acres (Lacustrine System).

There are nine Cowardin classes possible in the Palustrine System; four common types were sampled in the Gallatin Valley. Also, one Riverine System was described. See a complete NWI legend here.

What is the Hydrogeomorphic Approach?

The (Hydrogeomorphic (HGM) Approach)was developed to evaluate impacts on wetland function as part of the permitting procedure required by Section 404 of the Clean Water Act.

Digital wetland information can be enhanced with the addition of HGM descriptors for landscape position, landform, water flow path, and waterbody type (LLWW). LLWW descriptors bridge the gap between wetland types and wetland functions.

It is based on the three fundamental factors:

1) landscape position, such as depression or riverine

2) water source and transport, such as precipitation or groundwater

3) hydrodynamics, such as the unidirectional flow of a sloping drainway or the vertical flow in a depression.

The HGM method recognizes functional differences between types of wetlands and their location in the watershed, and ranks them accordingly. For example, a depressional wetland high in a watershed will better perform the function of water storage than a floodplain wetland.