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.