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In the 1780's, the present boundaries of the Nation as we know it today were composed of states, state-claimed areas, organized and unorganized territories, and foreign lands. For these reasons historical acreage estimates are based only partially on colonial or state historical records. In addition, land use records tracing conversion of lands by use categories, drainage statistics, and information on the extent of hydric soils (drained and undrained), in combination with historical wetland acreage data, have been used to estimate the original wetland acreage for each state (refer to Table 1).

Data on existing (1980's) wetland acreage also must be interpreted with caution. For some states, the wetlands have been mapped for the entire state by the National Wetlands Inventory, and acreage summary reports are available detailing the extent of wetlands. However, for those states where wetlands are not completely mapped or where acreage summaries are not yet compiled, an accurate accounting of wetland acreage is not always available. For some states, there are conflicting data sets reflecting inconsistencies in inventory terminologies or techniques, inadequate inventory data, or simply outdated information. In several cases, published documentation on the extent of wetlands amounts to little more than speculation. In these instances, an effort was made to assess the validity of the information and reconcile acreage statistics with the best national or regional data sets available to determine statewide totals.

Additionally, the current status of wetlands in the United States is constantly changing. It is estimated that, on average, over 60 acres of wetlands have been lost every hour in the lower 48 states during this 200-year timespan. While some state and Federal agencies are attempting to restore welands in certain parts of the country, restoration falls far short of this loss rate. In all instances, data sources for state estimates have been referenced.