Raise water level to restore degraded freshwater marshes
Overall effectiveness category Trade-off between benefit and harms
Number of studies: 5
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Background information and definitions
This action involves one-off action to raise the water level/table in degraded marshes, to a depth that should support emergent vegetation. This means that intervention should (a) occur at one point in time, after which the water level is not actively managed, and (b) must affect a marsh that is drier than normal but that is still recognisable as, or retains substantial characteristics of, the target habitat.
Specific techniques to raise water levels include: blocking drainage ditches (using sediment, rocks, plastic dams, wooden dams or vegetation); building raised embankments, berms or levees to retain water; switching off drainage pumps; ceasing groundwater extraction; installing or widening culverts (e.g. under roads and railways, to increase water flow into focal site); removing dams upstream of the focal site; and reprofiling or diverting river channels to raise the water level on floodplains. All of these techniques aim to make soils saturated or flooded, or make them saturated or flooded for longer, so they can support emergent wetland vegetation. The resulting water level may be stable or fluctuating, and may create permanently or seasonally flooded wetlands. Sediment inputs may also increase in line with water inputs.
Caution: This action may have negative effects on habitats elsewhere in the catchment. For example, removing dams upstream of a focal site could drain wetlands or aquatic habitats upstream of the dam. There may also be conflicts with water needs of human populations that need to be managed.
Related actions: Raise water level to restore/create marshes from other land uses; Actively manage water level; Manage water level to control problematic plants; Reprofile/relandscape or Remove surface soil/sediment, both of which can lower the ground surface towards the water table; Raise water level to complement planting; Restore/create marshes or swamps using multiple interventions, often including water level manipulations.
Supporting evidence from individual studies
A replicated study in 1949–1957 in a freshwater wetland in Minnesota, USA (Harris & Marshall 1963) reported that the effects of reflooding on emergent plant abundance depended on the water level and species. Statistical significance was not assessed. In areas with deep water (>15 inches in summer, after reflooding), the density of all emergent plant species declined (e.g. softstem bulrush Scirpus validus: 7.1 stems/ft2 after 1 year of reflooding then 0 stems/ft2 after four years of reflooding; cattails Typha spp.: 0.8 stems/ft2 vs 0.4 stems/ft2). In areas with shallow water (0–10 inches in summer, after reflooding), the density of softstem bulrush and spikesedge Eleocharis palustris declined (9.6–10.3 stems/ft2 after one year vs 0.1–0.3 stems/ft2 after four years) whilst the density of cattails and sedges Carex spp. increased (1.0–1.5 stems/ft2 vs 2.2–2.5 stems/ft2). Methods: At some point between 1949 and 1957, water levels were raised in four separate wetland pools that had been drawn down for the previous 1–5 years. Vegetation was surveyed between one and four years after reflooding, in stands initially dominated by each plant species but with different post-reflooding water depths.Study and other actions tested
A replicated, site comparison study in 1993–1998 involving 12 dune slacks in the Netherlands (Grootjans et al. 2001) reported that after stopping groundwater extraction (along with removing topsoil and resuming grazing), the slacks developed plant communities with habitat-characteristic species, and more species than mature, unmanaged slacks. Statistical significance was not assessed. Restored slacks developed plant communities, the overall composition of which changed over time (data reported as a graphical analysis). After five years, restored slacks contained 76–108 plant species overall and 48–86 species/100 m2. This included species characteristic of dune slacks (5–11 species/100 m2) and nutrient-rich marshes (2–11 species/100 m2) alongside other wetland and upland species. In each slack, total vegetation cover was always <50% and only two individual species – creeping willow Salix repens and bushgrass Calamagrostis epigejos – ever had cover >1%. For comparison, during the second year of the study, mature slacks contained 12–39 plant species/m2 (data not reported for other outcomes). Methods: Dune slacks are low-lying areas amongst dunes. Eight degraded slacks (stabilized and covered with undesirable, mature vegetation) were restored. In 1993, groundwater extraction was stopped. Vegetation and topsoil were also stripped, completely or partially, from each slack. In 1995, grazers (a “small herd” of cattle and ponies) were reintroduced to seven slacks. The study does not distinguish between the effects of these interventions. Vegetation was surveyed in at least five of the restored slacks (spring or summer 1994–1998) and four mature slacks (spring 1994): species across the whole of each slack; species and cover in five comparable 100-m2 plots/slack.Study and other actions tested
A replicated, before-and-after study in 1998–2008 of two marshes on a floodplain in Florida, USA (Toth 2010) reported that raising water levels by filling drainage/flood control channels had mixed effects on cover of plant groups, but consistently reduced overall plant species richness and vegetation cover. Unless specified, results summarized for this study are not based on assessments of statistical significance. Before intervention, both marshes were dominated by wetland-characteristic grasses (24–52% cover) with some vegetation characteristic of broadleaf marshes (12–30% cover). Over the first 4–6 years after raising water levels, one marsh remained dominated by wetland-characteristic grasses (18–50% cover). The other became dominated by broadleaf marsh vegetation (11–68% cover). In subsequent years, both marshes were dominated by a mix of Peruvian water primrose Ludwigia peruviana (9–70%) and broadleaf marsh vegetation (4–34% cover). Total vegetation cover and species richness were variable over time, but often lower after intervention (49–97% cover; 7–20 species/100 m2) than before (77–93% cover; 16–26 species/100 m2). Plant diversity was statistically similar before and after intervention in both marshes (data not reported). Methods: Between 1999 and 2001, the water level was raised in two degraded marshes in Sections A and C of the Kissimmee River floodplain. This was achieved by “eliminating” a drainage ditch (one marsh) and dechannelizing the river (other marsh). Plant species and their cover were surveyed before intervention (from summer 1998) and for approximately seven years after (until summer 2008), in three 100-m2 plots/marsh.Study and other actions tested
A before-and-after study in 1999–2011 of a floodplain in Hokkaido, Japan (Nakamura et al. 2014) reported that following restoration of the natural meandering river course, the area of emergent herbaceous vegetation increased. Statistical significance was not assessed. Marshes covered approximately 50 ha of the floodplain around 10 years before restoration began, then 77 ha around five years after restoration began. More specifically, there were increases in the area of stands dominated by knotweed Polygonum thunbergii (before: 0 ha; after: 36 ha) and stands dominated by common rush Juncus effusus (before: 0 ha; after: 9 ha) – mostly in a recently (<9 months old) relandscaped area. In contrast, there were decreases the area of mixed common reed Phragmites australis and sedge Carex spp. stands (before: 27 ha; after: 19 ha) and wet meadows dominated by reed canarygrass Phalaris arundinacea (before: 22 ha; after: 13 ha). Methods: The Kushiro River was channelized and straightened in the 1970s. Between 2007 and 2011, its natural course was restored (2007–2010: former meandering channel excavated and reflooded; 2011: flood embankments removed and straightened section backfilled). Flooding frequency increased in the surrounding floodplain, and the water table rose to near the ground surface. Vegetation was mapped before (1999) and after (2011) restoration, from aerial photographs and with field surveys.Study and other actions tested
A replicated, randomized, paired, controlled, before-and-after study in 2011–2012 of marshes within a pine forest in North Carolina, USA (Aschehoug et al. 2015) found that damming to raise the water table limited understory vegetation cover, but had no significant effect on sedge cover. In rewetted plots, there was no change in total understory vegetation cover (42% one month before thinning and 42% one year after). However, in plots that remained drained, understory vegetation cover increased (from 35 to 58%). Total sedge Carex spp. cover increased by statistically similar amounts in rewetted plots (from 11 to 17%) and drained plots (from 6 to 15%). Methods: In May 2011, sixteen 30 x 30 m plots were established (in four blocks of four) on tree-colonized marshes within a pine forest. Maintenance of open marsh had been restricted by fire suppression and the extirpation of beavers Castor canadensis. Dams were installed on the downstream edge of eight plots (two/block), raising the water table. About a third of each plot was flooded. The other eight plots remained drained. Trees were also thinned in four rewetted and four drained plots. Vegetation cover was visually estimated one month before (April 2011) and one year after (April 2012) intervention.Study and other actions tested
Where has this evidence come from?
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This Action forms part of the Action Synopsis:Marsh and Swamp Conservation
Marsh and Swamp Conservation - Published 2021
Marsh and Swamp Synopsis