Lower water level to restore/create freshwater marshes from other land uses
Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
Background information and definitions
This action involves one-off action to lower the water level and restore or create marshes from other land uses, i.e. in areas that do not retain substantial characteristics of the target habitat. By definition, these other land uses will always be aquatic habitats such as reservoirs or lakes. The lowered water level should not depend on continued intervention (e.g. pumping). Specific techniques to reduce water levels include removing dams downstream, or switching off pumps that add water to a focal site. This action includes water level reductions to any depth that could, in theory, support emergent wetland vegetation.
Caution: This action may have negative effects on habitats elsewhere in the catchment. For example, removing dams could flood marshes, swamps or upland habitats downstream. There may also be conflicts with water needs of human populations that need to be managed.
Related actions: Lower water level to restore degraded marshes; Backfill canals or trenches; Actively manage water level; Reprofile/relandscape, which may involve raising the ground surface towards or above the water table; Lower water level to complement planting.
Supporting evidence from individual studies
A replicated, before-and-after study in 1949–1957 in a freshwater wetland in Minnesota, USA (Harris & Marshall 1963) reported that following drawdown of water levels, emergent wetland vegetation colonized the site. Over five years of drawdown, stands of tall emergent plants like softstem bulrush Scirpus validus, cattails Typha spp. and sedges Carex spp. developed on approximately 5,000 acres of 12,000 acres that were previously open water. Elsewhere, exposed mudflats were colonized by species such as marsh fleabane Senecio congestus and red goosefoot Chenopodium rubrum (area not quantified). The study suggested several related factors that affected the type of vegetation that developed, e.g. month of drawdown, soil type (mineral or peat), speed of drying, seed availability, and presence of algal mats. Herbaceous wetland communities present in the first year of drawdown were largely replaced by upland weeds, then woody species, over the following four years. Methods: At some point between 1949 and 1957, water levels were lowered in seven separate wetland pools to stimulate growth of emergent and moist-soil wetland vegetation. Two pools supported islands of emergent vegetation before drawdown. Observations were made after 1–5 years of drawdown in each pool (further details not reported).Study and other actions tested
A before-and-after study in 1987–1992 of a freshwater lake in the Netherlands (ter Heerdt & Drost 1994) reported that following drawdown of the water level, emergent wetland vegetation colonized. Cover of vegetation overall and of individual plant species depended on elevation and length of drawdown. For example, the highest, driest zone (exposed from March/April 1987) developed 63% total vegetation cover after one growing season. It was dominated by broadleaf cattail Typha latifolia (53% of total). After four years, total cover was 103% and the dominant species was great willowherb Epilobium hirsutum (63% of total). The lowest, wettest zone (exposed from April/July 1988) developed 16% total cover after one growing season. It was dominated by swamp ragwort Senecio congestus (87% of total). After four years, total cover was 36% and the dominant species was toad rush Juncus bufonius (28% of total). Zones at intermediate elevation developed 87–109% total cover after four years, dominated by common reed Phragmites australis (51–94% of total). Methods: The water level of Groteplas Lake was lowered from 1987, gradually exposing formerly flooded areas. The highest shoreline zones (with some islands of emergent vegetation before drawdown) were exposed in 1987. The lowest zones (no emergent vegetation before drawdown) were exposed in 1988. Cover of each plant species and vegetation overall were recorded along transects in exposed areas after 1–4 growing seasons (September/October 1987–1992).Study and other actions tested