Raise water level to restore degraded freshwater marshes

How is the evidence assessed?
  • Effectiveness
    not assessed
  • Certainty
    not assessed
  • Harms
    not assessed

Study locations

Key messages

  • Five studies evaluated the effects, on vegetation, of raising the water level to restore degraded freshwater marshes. There were three studies in the USA and one in each of the Netherlands and Japan.

VEGETATION COMMUNITY

  • Overall extent (1 study): One before-and-after study of a floodplain in Japan reported that the area covered by marsh vegetation was higher five years after dechannelizing a river than 10 years before.
  • Community types (1 study): One before-and-after study of a floodplain in Japan reported changes in the area covered by different marsh plant communities over five years after dechannelizing a river compared to 10 years before.
  • Community composition (1 study): One replicated study of dune slacks in the Netherlands reported changes in the overall plant community composition after stopping groundwater extraction (along with other interventions).
  • Overall richness/diversity (2 studies): One replicated, site comparison study of dune slacks in the Netherlands reported that overall plant species richness was greater in restored slacks (groundwater extraction stopped five years previously, along with other interventions) than in mature unmanaged slacks. One replicated, before-and-after study of floodplain marshes in the USA reported that total plant species richness tended to be lower over nine years after raising the water table than before, but that there was no significant difference for diversity.
  • Characteristic plant richness/diversity (1 study): One replicated study of dune slacks in the Netherlands simply quantified the richness of characteristic plant species – typical of dune slacks or nutrient-rich marshes – over five years after stopping groundwater extraction (along with other interventions).

VEGETATION ABUNDANCE

  • Overall abundance (3 studies): One replicated, before-and-after study of floodplain marshes in the USA reported that total vegetation cover tended to be lower over nine years after raising the water table than before. One replicated, randomized, paired, controlled, before-and-after study of freshwater marshes in the USA found that damming to raise the water table prevented increases in understory vegetation cover over the following year. One replicated study of dune slacks in the Netherlands simply quantified total vegetation over five years after stopping groundwater extraction (along with other interventions). Cover never exceeded 50%.
  • Herb abundance (1 study): One replicated, randomized, paired, controlled, before-and-after study of freshwater marshes in the USA found that damming to raise the water table had no significant effect on cover of sedges Carex There were similar increases in dammed and undammed marshes over one year.
  • Characteristic plant abundance (1 study): One replicated, before-and-after study of floodplain marshes in the USA reported changes in the cover of wetland- and habitat-characteristic plant species over nine years after raising the water table.
  • Individual species abundance (3 studies): Three studies quantified the effect of this intervention on the abundance of individual plant species. For example, one replicated, before-and-after study in the USA reported that rewetted floodplain marshes became dominated by a non-native wetland shrub, approximately 4–9 years after raising the water table. One replicated study of a freshwater wetland in the USA reported that the effects of reflooding on the density of emergent plant species depended on the species and water level.

VEGETATION STRUCTURE

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. 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
  2. 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
  3. 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
  4. 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
  5. 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
Please cite as:

Taylor N.G., Grillas P., Smith R.K. & Sutherland W.J. (2021) Marsh and Swamp Conservation: Global Evidence for the Effects of Interventions to Conserve Marsh and Swamp Vegetation. Conservation Evidence Series Synopses. University of Cambridge, Cambridge, UK.

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Marsh and Swamp Conservation

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation
Marsh and Swamp Conservation

Marsh and Swamp Conservation - Published 2021

Marsh and Swamp Synopsis

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