Home 2. Estimate Wetland Loss

Estimate Wetland Loss

On a national basis, it is estimated that around 53% of wetlands present at the time of European settlement in the early 1600s have since been lost for the conterminous U.S. (Dahl and Johnson, 1991). Six states lost 85% or more of their original wetland acreage, while twenty-two lost 50% or more (Dahl, 1990). If wetland losses from agriculture and urban development have been significant in your community, it is likely that your community has also lost valuable functions provided by these wetlands, such as flood storage or water quality.  Historic wetland data can provide some insight into the extent of wetland losses and can help to make the case for protecting remaining wetlands. A more detailed mapping exercise can also be completed to estimate the extent, types and functions associated with historic wetlands. These maps can also provide the basis for identifying potential sites for wetland restoration to replace lost wetland functions.

For some communities, historic maps and other data that provide an estimate of historic wetland coverage may be readily available.  If this is the case, all that may be required is scanning, digitizing and/or geo-referencing old maps, or weeding through a large dataset to pull out and compile data for your area of interest. Some potential sources of historic wetlands data include:

  • U.S. Fish and Wildlife Service (USFWS) Status and Trends reports. Dahl (1990) provides state-wide estimates of wetland losses from the 1780s to the 1980s. Other reports provide state or region-wide estimates of wetland losses documented since the 1950s.
  • Old U.S. Geological Survey maps
  • General Land Office Survey Maps and Notes, which are surveys conducted of newly acquired U.S. territories by the General Land Office (GLO) that was formed in 1812.
  • Statistics on acres and types of wetlands filled under Section 404 permits can be obtained from your U.S. Army Corps of Engineers (USACE) District Office.
  • Historic maps and vegetation surveys and interviews with botanists familiar with local historical plant communities.  Old maps may be available from local libraries or natural heritage agencies.
  • For some areas of the country, the NWI contains historic wetlands map Information. Information about the wetland types, vegetation, regional and temporal conditions and geographic features are captured in a historic map document. This document can be accessed by selecting a wetland polygon on the Wetlands Mapper and then clicking on the link next to ‘Historic Map Info’ in the pop-up window.
  • The NRCS Natural Resources Inventory data, a statistical survey of land use and natural resources conditions and trends on non-federal lands, can provide a general estimate of wetland losses on a state or national level for specific time periods.
  • NRCS Wetland Determinations (aka “Swampbuster” maps) were used to determine compliance with swampbuster provisions in the 1985 Farm Bill. These wetland determinations are available as paper maps only for individual sites and contain wetlands classified as Prior Converted (PC) or Farmed Wetlands (FW). Prior converted wetlands are those converted to a non-wetland state prior to 1985. Farmed wetlands are those manipulated and planted prior to 1985, but that still meet wetland criteria. Digitizing these wetlands may be useful to supplement your map of historic wetlands.

Note that the above sources may simply provide an estimate of historic wetlands acreage that can then be compared to current wetland acreage to quantify loss.  Specifics on wetland types, locations, and functions may not be available from these sources. Also, keep in mind that differences in the quality and resolution of historic and current wetland datasets will influence the results; for instance, the data may show “no change” or even a net “gain” in wetlands due to these inconsistencies. If a historic wetland map does not exist for your community, has known inaccuracies, or is of a relatively small scale (e.g., smaller than 1:40,000), using wetland indicator layers to map historic wetlands provides a method to generate a detailed map of historic wetlands.

     

 

CASE STUDY - Seven Mile Creek Watershed in Minnesota

Seven Mile Creek is a 36.8 mi2 agricultural watershed located in south-central Minnesota in the Minnesota River Basin.  The watershed is dominated by flat agricultural fields with numerous small depressions. As of 2002, 81% of the watershed land use was cropland. A study was conducted to determine the extent of wetland loss to cropland, engineered surface and sub-surface drainage modifications, and general cropping system shifts by analyzing aerial photos from seven different time periods dating back to 1938.

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CASE STUDY - Wetlands and Deepwater Habitats of the Nanticoke River Watershed

The Nanticoke River watershed is a tributary to the Chesapeake Bay, located in Delaware and Maryland and is approximately 2,070 km2 in size. The two states worked cooperatively to develop a watershed-based strategy for wetland conservation and restoration.  The goal was to produce an inventory and analysis of historic wetlands and their functions for the Nanticoke River watershed and to compare those findings to present-day conditions.

 

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Resources for Estimating Wetland Loss

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Digitize
The process of converting features on a paper map into digital format using a trace methodology, which results in the creation of a spatial dataset.
Ecotone

A transition area between two adjacent, but different plant communities.

Indirect Wetland Impacts
Impact to wetlands caused by inputs of stormwater and pollutants generated by land development or other activities within the wetland CDA.
Direct Wetland Impacts
Wetland loss or degradation resulting from activities that occur within wetlands, such as dredging, filling and draining.  Activities that cause direct impacts are largely regulated through the federal and state wetland permitting process.
Stormwater Treatment Practices

A structural or non-structural practice designed to temporarily store or treat stormwater runoff in order to mitigate flooding, reduce pollution, and provide other amenities (also called a Best Management Practice – BMP).

Hydrogeomorphic
Factors that influence how wetlands function, including geomorphic setting, water source, and hydrodynamics.
Hydrogeomorphic
Factors that influence how wetlands function, including geomorphic setting, water source, and hydrodynamics.
Sinks
A cell or set of spatially connected cells that cannot be assigned flow direction in a raster elevation dataset. This can occur when all neighboring cells are higher than the processing cell or when two cells flow into one another. Sinks can indicate areas where water is likely to pond, but can also be an error in the dataset.
Facultative Wetland Plants
Species that usually occur in wetlands (approximately 67% - 99% probability), but also occur in non-wetland areas (approximately 1% - 33% probability).
Obligate Wetland Plants
Species that occur almost always in wetlands under natural conditions (greater than 99% probability), but which may also occur rarely in non-wetlands (less than 1% probability).
Interferometric Synthetic Aperture Radar (IFSAR)
A radar technique that uses two or more synthetic aperture radar (SAR) images to generate surface elevation using differences in the phase of waves returning to the satellite or aircraft.
Interferometric Synthetic Aperture Radar (IFSAR)
A radar technique that uses two or more synthetic aperture radar (SAR) images to generate surface elevation using differences in the phase of waves returning to the satellite or aircraft.
Light Detection and Ranging (LiDAR)

A remote sensing technique that measures properties of pulsed laser light reflected from objects to determine their position, velocity, and other information.

Light Detection and Ranging (LiDAR)

A remote sensing technique that measures properties of pulsed laser light reflected from objects to determine their position, velocity, and other information.

Light Detection and Ranging (LiDAR)

A remote sensing technique that measures properties of pulsed laser light reflected from objects to determine their position, velocity, and other information.

Digital Elevation Model (DEM)
A digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals.
Digital Elevation Model (DEM)
A digital file consisting of terrain elevations for ground positions at regularly spaced horizontal intervals.
Hyperspectral Data

Information collected and processed from across the electromagnetic spectrum. Spectral signatures (unique “fingerprint” left by specific objects) enable identification of materials that make up a scanned object.

Remote Sensing
Gathering and recording information about objects without actual contact through the use of such techniques as photography, infra-red imagery, and radar.
Hydrophytes
A plant that grows wholly or partially submerged in water.
Blackspots
Areas on aerial photos that show up as dark blue, dark grey, or black and are indicative of saturated soil conditions.
Stereoscopic
The ability to see three dimensionally by using two views of a single object from slightly different positions typically through the use of an optical aid known as a stereoscope.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Hydric Soils
Soils that are saturated, flooded, or ponded for a long enough period during the growing season to develop anaerobic conditions in the upper soil horizons.
Geographic Information Systems (GIS)

A system that integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information.

Geographic Information Systems (GIS)

A system that integrates hardware, software, and data for capturing, managing, analyzing, and displaying all forms of geographically referenced information.

Digitize
The process of converting features on a paper map into digital format using a trace methodology, which results in the creation of a spatial dataset.
Minimum Mapping Unit

The minimum size or dimensions for features to be mapped as lines or areas for a given map scale.