Wetland Delineations: Types, Steps, and Regional Differences
Since 1991, Minnesota has regulated a “no net loss” of wetland area within the state. Delineation is the process by which boundaries of existing wetlands are defined and area is measured – a critical component of the mitigation banking system of credit offsets.
Landbridge Ecological can help property owners define and measure wetland areas.
Purpose of a Delineation
Minnesota’s Wetland Conservation Act (WCA) of 1991 mandates no net loss of wetland area in the state. The act provides for wetland mitigation banking, a system of restored and created wetlands that can offset (or mitigate) impacts on natural wetlands elsewhere – for example, destruction of wetlands through draining or filling for agriculture or construction activities. The cost for offsetting activity that impacts wetlands is administered through wetland bank credits and associated fees.
Because of the close regulation of wetlands through the WCA, the area means money. The boundary of a wetland that may be impacted by development must be precisely determined and the area contained within that boundary precisely measured. The process of determining this boundary is wetland delineation. Wetland delineators follow guidelines developed in 1987 by the Army Corps of Engineers, with subsequent regional updates.
Wetland Delineation Can Be Done in One of Three Ways:
Level 1: Off-Site Delineation uses aerial imagery, topographical maps, soil maps, and the National Wetland Inventory to determine the likely boundary of a wetland. This method can be used, for example, when it’s not critical to know the exact boundary line or when the proposed impact on a wetland will be only temporary.
Level 2: On-Site Delineation is the default method when the exact boundary of a wetland is needed for regulatory purposes – for example, when the developer will have to offset the destruction of a wetland through purchase of wetland bank credits. It will also be preferred for complex wetlands with irregular boundaries. The steps for collecting the field data necessary for an on-site delineation are spelled out below.
Level 3: Combination of Levels 1 and 2 can be used when only part of a wetland’s boundary is in question or under review. When part of a wetland's boundary is easily determined by off-site inference from maps, or is not contested, or is not likely to be impacted by development, that part can be delineated using Level 1. The rest of the boundary will require the more precise methods of Level 2 delineation.
A point on the landscape is determined to be within a wetland if it fulfills three criteria:
Hydrophytic Vegetation
Wetland Hydrology
Hydric Soils
Steps of a Delineation: Vegetation, Hydrology, and Soils
A point on the landscape is determined to be within a wetland if it fulfills three criteria: 1) hydrophytic (water-loving) vegetation present, 2) wetland hydrology present, 3) hydric soils present.
The delineator will use cues visible on the ground and off-site “scouting” from the study of maps to determine the expected wetland boundary. A transect perpendicular to the expected wetland boundary is run, and points along the transect are sampled. The first sample point will be clearly upland, the last sample point will be clearly wetland, and intermediate points will be sampled at appropriate intervals in order to determine precisely where the transition from upland to wetland occurs. Additional transects, roughly parallel to the first, are run along the expected boundary. The distance between transects must be no greater than half a mile, but may be as close as 200 or even 50 feet, depending on the complexity of the wetland and the degree of contention about the outcome of the delineation.
Hydrophytic Vegetation
The delineator first examines the vegetation at the sample point. Each plant species is rated with a wetland indicator status according to its general preference for wetland or upland habitat: OBL for “obligate” species occurring only in wetlands, FACW for “facultative wetland” species occurring usually in wetlands, FAC for “facultative” species equally likely to occur in both habitats, FACU for “facultative upland” species occurring usually in uplands, and UPL for “upland”– only species.
The dominant species at each sample point are inventoried. A meticulous scoring system takes into account each species present, its wetland/upland rating, and its percent area on site. The scoring determines whether the overall plant community is more wetland (“hydrophytic vegetation”) or more upland.
How big an area at each point is sampled differs depending on the kind of vegetation. For herbaceous vegetation, shrubs, and tree regeneration, a circle with a 5-foot radius from the plot center is sampled. For woody vines and trees, the circle is 30 feet in radius.
Wetland Hydrology
This is the fun part of a wetland delineation. Obviously, if under normal conditions a sample point is under water or has saturated soil, it can be said to exhibit wetland hydrology. But hydrology can differ from year to year or within the growing season, so the practiced observer relies on a suite of features to determine if a given point on the landscape can be classified as wetland. Examples include water lines on trees, crayfish burrows, plants with “morphological adaptations” such as buttressed trunks, drift lines with deposits of silt or debris, and historical records.
Some of these features are considered “primary indicators” and some are considered “secondary indicators”. A certain number of primary and secondary indicators is necessary in order to classify the hydrology as wetland.
Hydric Soils
In certain cases, the delineation can stop after vegetation and hydrology are evaluated—for example, if the vegetation is overwhelmingly hydrophytic and the transition from upland to wetland is obvious on the landscape. In most cases, however, soils will also have to be evaluated. This requires specialized knowledge.
Many wetlands tend to accumulate thick layers of organic matter, since decomposition is slow in waterlogged, oxygen-deprived conditions. A soil characterized by thick layers of organic matter at various stages of decomposition is easily identified as hydric. But mineral soils can also show evidence of periodic saturation pervasive enough to merit their classification as hydric: these often have to do with transformations undergone by iron minerals. The wetland delineator relies on texture, color, and even odor (the “rotten egg” smell from hydrogen sulfide, a byproduct of microbial activity in wetland soils). The delineator may also need to take into account the “parent material” from which the soil formed.
Regional Differences
Because of differences in climate, geomorphology, and vegetation across the landscape, important criteria for wetland delineation may vary by region. The Army Corps of Engineers recognizes ten regions across the United States and its territories. Minnesota is the intersection of three of these regions: Great Plains, Midwest, and North Central–Northeast.
A given plant species may have a different wetland indicator status in different regions: blue lobelia is an obligate wetland species in the Great Plains and Midwest regions but a facultative wetland species in the North Central–Northeast region. Heart-leaved Alexanders are more likely to be found in wetlands in the Great Plains (where their status is FAC for facultative) than in the other two regions (where their status is FACU for facultative upland).
Likewise, field indicators of wetland hydrology may differ in significance and prevalence from region to region. For example, a sparsely vegetated concave surface is a primary indicator of wetland hydrology in the Midwest and North Central–Northeast regions; but it’s only a secondary and thus less decisive indicator in the drier Great Plains region, where vegetation generally tends to be sparser.
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Contact MPI today with your ecological restoration and mitigation banking project needs. Our team is ready with answers and expert services to bring your vision to life.
Mitigation Partners, Inc. Founders Dax Dickson & Tory Christensen
Sources
Delineation Guidance and Resources: http://bwsr.state.mn.us/delineation-guidance-resources
Wetland Conservation Act—Wetland Delineations: Choosing the Appropriate Method. BWSR Technical Guidance, July 1, 2010
Delineational Manuals: http://bwsr.state.mn.us/delineation-manuals
United States Army Corps of Engineers. 1987. Corps of Engineers Wetland Delineation Manual. Wetlands Research Program Technical Report Y-87-1. Vicksburg, Miss.: U.S. Army Corps of Engineers, Environmental Laboratory.
United States Army Corps of Engineers. 2010. Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Great Plains Region, version 2.0. (ERDC/EL TR-10-1) Vicksburg, Miss.: U.S. Army Corps of Engineers, U.S. Army Engineer Research and Development Center.
United States Army Corps of Engineers. 2010. Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Midwest Region, version 2.0. (ERDC/EL TR-10-16) Vicksburg, Miss.: U.S. Army Corps of Engineers, U.S. Army Engineer Research and Development Center.
United States Army Corps of Engineers. 2012. Regional Supplement to the Corps of Engineers Wetland Delineation Manual: Northcentral and Northeast Region, version 2.0. (ERDC/EL TR-12-1) Vicksburg, Miss.: U.S. Army Corps of Engineers, U.S. Army Engineer Research and Development Center.
