Wetland restoration is the repair or rebuilding of a degraded or former wetland's physical, chemical and biological characteristics in order to restore its natural functions. These functions include flood control and the maintenance of biodiversity.
Wetlands were formerly widely regarded as a waste of land, a nuisance, and even a health hazard. They have been degraded and destroyed perhaps more than any other type of ecosystem, through draining, contamination and filling. Some sources estimate that close to 90 percent of the world's wetlands has been lost during the past three hundred years, with most of this occurring after 1900. Fortunately, this situation has been changing and there is now growing awareness of the great importance of wetlands, increasingly accompanied by legislation and other efforts to protect, and in some cases restore, them — or even create new wetlands.
Before beginning a restoration project, it is important to take into consideration the characteristics of the particular wetland and its surrounding area. These include the type of wetland (e.g., swamp, marsh, fen, bog or estuary), water (inflows, outflows, levels as well as modifications such as drains and culverts), underlying substrate type (e.g., peat, mineral or some combination of both), vegetation types (including both original and invasive trees, rushes, sedges, mosses and rare species), animals (both original and invasive insects, amphibians, fish, birds, mammals, etc.), human-created features (roads, buildings, fences, pipelines, drains, etc.), and historic and cultural artifacts (including possibly unknown underground ones).
It is also important to consider the procedures and costs for monitoring the progress of the restoration and for maintenance as well as issues of encroachment and future uses by humans. A major part of maintenance is the removal of invasive species.