Actively manage water level: freshwater marshes

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

Study locations

Key messages

  • Ten studies evaluated the effects, on vegetation, of active water level management in freshwater marshes. Eight studies were in the USA. One study was in Cameroon and one study was in the Netherlands.

VEGETATION COMMUNITY

  • Community composition (1 study): One before-and-after study in the USA found that directly pumping water into drained marshes and wet meadows generated plant communities characteristic of wetter conditions. This change was reversed in some plots when the pump output was moved further away from the focal wetlands.
  • Relative abundance (2 studies): One replicated, randomized, controlled study of freshwater marshes in the USA reported that irrigated and non-irrigated marshes supported a similar relative abundance of the most common plant species. One before-and-after study on a floodplain in Cameroon found that the relative abundance of some key plant species changed over four years after restoring wet-season flooding. There was also an increase in the cover of perennial relative to annual herbs.
  • Overall richness/diversity (4 studies): One before-and-after study of a marsh/swamp in the USA found that overall plant diversity was higher in the autumn following a managed flood/drawdown than in the autumn before. Two before-and-after studies of marshes and wet meadows in the USA reported that plant species richness and/or diversity declined over 5–6 years of water level management (fluctuation or water addition). One study in the USA simply reported the number of plant species that colonized a floodplain, over three weeks after lowering the river level.

VEGETATION ABUNDANCE

  • Overall abundance (3 studies): One study of riparian moist/wet meadows in the USA reported that vascular plant biomass increased in two of three meadow types, over the second year of artificially augmented streamflow. Meanwhile, vascular plant cover declined in two of three meadow types. Two studies in the Netherlands and the USA simply quantified overall vegetation abundance after 1–9 growing seasons of active water level management (sometimes along with other interventions).
  • Characteristic plant abundance (2 studies): Two before-and-after studies of marshes and wet meadows in the USA reported increases in abundance of some individual wetland- or habitat-characteristic species over 5–6 years of water level management (fluctuation or water addition).
  • Moss abundance (1 study): One study of riparian moist/wet meadows in the USA reported that moss cover did not significantly change in two of three meadow types, over the second year of artificially augmented streamflow. It declined in the other meadow type.
  • Individual species abundance (7 studies): Seven studies quantified the effect of this action on the abundance of individual plant species. For example, one before-and-after study of a marsh/swamp in the USA reported mixed effects of a managed flood/drawdown on species cover, including increased cover of Pacific willow Salix lucida and reduced cover of reed canarygrass Phalaris arundinacea. One controlled study of freshwater marshes in the USA found that irrigated marshes developed a greater biomass of pink smartweed Polygonum pensylvanicum, after one growing season, than marshes that were left dry.

VEGETATION STRUCTURE

  • Vegetation height (1 study): One replicated, randomized, controlled study of freshwater marshes in the USA reported that four common plant species were taller in irrigated than non-irrigated marshes.

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 study in 1985–1987 of 18 riparian moist/wet meadows along one stream in Wyoming, USA (Henszey et al. 1991) found that the effect of augmenting streamflow on vegetation abundance depended on meadow wetness, plant species and abundance metric. For example, over the second year of augmented streamflow, above-ground vascular plant biomass significantly increased at two of three moisture levels (from 412–433 g/m2 to 517–532 g/m2), whilst vascular plant cover significantly declined at two of three moisture levels (from 22–23% to 15%). The study also reported data on the abundance of individual plant species (see original paper). For example, biomass of the dominant sedges Carex spp. increased significantly in two of three comparisons (from 262–337 g/m2 to 386–456 g/m2) with a similar trend in the other (from 379 to 452 g/m2). Moss cover did not significantly change in two of three comparisons (from 3–27% to 8–24%) but declined in the other (from 29% to 4%). Methods: From August 1985, additional water was released into a previously ephemeral stream, increasing the flow and raising the water table in streamside meadows. Vegetation abundance was surveyed in 18 meadows (six moist, six moist-wet, six wet). Biomass was dried before weighing. Surveys took place in August–October 1985–1987, starting shortly after streamflow augmentation began. Large grazing mammals were excluded from some of the surveyed meadows.

    Study and other actions tested
  2. A replicated, randomized, controlled study in 1990 in nine ephemeral freshwater marshes in California, USA (Mushet et al. 1992) reported that irrigated marshes and non-irrigated marshes supported a similar relative abundance of common plant species, but that irrigation increased the height of four species and increased the biomass of two. Results summarized for this study are not based on assessments of statistical significance. All marshes were drained in spring. Four months later, the relative abundance of the most common plant species was broadly similar in irrigated and non-irrigated marshes: 49–62% of individual plants were pricklegrass Crypsis niliaca, 30–40% were sprangletop Leptochloa fasicularis, <5% were barnyardgrass Echinochloa crusgalli and <5% were swamp timothy Heleochloa schoenoides. However, all of these species were taller in irrigated marshes (3–29 cm) than in non-irrigated marshes (1–5 cm). Above-ground biomass of sprangletop and barnyardgrass was greater in irrigated marshes (0.1–0.7 g/plant) than non-irrigated marshes (<0.1–0.2 g/plant). Irrigation had little effect on the above-ground biomass of the other two species (irrigated: 0.1–0.2 g/plant; not irrigated: 0.1–0.2 g/plant). Methods: Nine adjacent experimental marshes were drained in April 1990 to simulate management for waterfowl. Six random marshes were irrigated by flooding (for <24 h): three in May only, and three in May and June. The other three marshes were not irrigated. In July–August, plants were counted and identified in five 0.25-m2 plots/marsh, and 40 plants/species/marsh were measured, cut, dried and weighed.

    Study and other actions tested
  3. A before-and-after study in 1993–1997 on a floodplain in northern Cameroon (Scholte et al. 2000) found that after releasing water from a dam to restore seasonal flooding, there were changes in the relative abundance of some plant species and groups. Of eight monitored plant species, three were significantly less dominant in the year after reflooding than in the year before (antelope grass Echinochloa pyramidalis, turf grass Ischaemum afrum and vetiver grass Vetiveria nigritana; data not reported). Over the four years after reflooding, there were increases in the relative cover of antelope grass (from 9% to 19%) and wild rice Oryza longistaminata (from 29% to 38%), and a decrease in the relative cover of wild sorghum Sorghum arundinaceum (from 26% to 16%). Meanwhile, there was an increase in the relative cover of perennial herbs (from 41% to 62%), but a decrease in the relative cover of annual herbs (from 58% to 34%). Methods: This summary is based on data from 108 permanent 36-m2 plots, spaced 0.5–1.0 km apart on a floodplain. Wet-season flooding of these plots (approximately August to December) had been restricted after a dam was built upstream in 1979. Wet-season flooding was restored from 1994 by releasing water from the dam. Vegetation was surveyed, in the dry season, before reflooding (1993) and in four years after (1994–1997).

    Study and other actions tested
  4. A study in 1995–1998 on the shore of a freshwater lake in the Netherlands (Coops et al. 2004) reported that following a managed sequence of drawdowns and floods, a band of emergent wetland vegetation developed. Initially, the study area was a sandy shoreline with little or no vegetation (not quantified). After three years, a 50-m-wide band of tall emergent vegetation had developed around and just below the drawdown water line. This included sea club rush Bolboschoenus maritimus, common reed Phragmites australis, cattails Typha spp. and bulrushes Schoenoplectus spp. Total above-ground biomass of these species reached 520–630 g/m2 in the most densely vegetated areas (data reported graphically). Brackish marsh vegetation developed at slightly higher elevations (not quantified). Methods: In 1995, a watertight exclosure was established around a 3-ha area on the shoreline of Lake Volkerak-Zoommeer. The lake was created from an estuary in 1987 and the water level had been held stable since. The water level was lowered throughout 1995 and 1996 (exposing a 60-m-wide band of shoreline), then raised each winter and lowered each summer (exposing a 20- to 30-m-wide band of shoreline). Parts of the study area were fenced to exclude waterbirds, but parts were left open. Each summer between 1995 and 1998, vegetation was surveyed along transects spanning the drawdown zone (i.e. perpendicular to shoreline).

    Study and other actions tested
  5. A controlled study in 1993 of four depressional freshwater marshes in Texas, USA (Haukos & Smith 2006) found that marshes kept wetter over the growing season supported greater above-ground biomass of pink smartweed Polygonum pensylvanicum, at the end of the growing season, than drier marshes. This was true for marshes kept permanently flooded to 3–5 cm depth (12,497 kg/ha), marshes kept permanently wet but not flooded (9,010 kg/ha), and marshes that fluctuated between flooded, wet and dry (6,816 kg/ha). In permanently dry marshes, with plants wilting during day, above-ground biomass of pink smartweed was only 4,706 kg/ha. Methods: Four marshes (6.1 ha average size) were kept saturated in spring to promote germination of pink smartweed. Between June and August, three marshes were managed with increased water levels and one marsh was left dry. In September, smartweed was cut from twelve 0.25-m2 quadrats/marsh. Seeds were removed, then the vegetation was dried and weighed.

    Study and other actions tested
  6. A before-and-after study in 2003–2004 of a freshwater wetland with marsh and swamp vegetation in Oregon, USA (Jenkins et al. 2008) found that following a managed flood/drawdown, plant diversity increased and there were changes in cover of individual plant taxa. Plant diversity was higher in the autumn after the flood/drawdown than in the autumn before (data reported as a diversity index). Of 21 plant taxa for which cover data were reported, 12 became more abundant, including knotweeds Polygonum spp. (before: 21%; after: 35%) and Pacific willow Salix lucida (before: 11%; after: 15%). The cover of seven taxa declined, including invasive reed canarygrass Phalaris arundinacea (before: 44%; after: 41%). The largest canarygrass declines occurred under regenerating tree canopies and in areas more deeply flooded during spring 2004 (see original paper for data). Methods: In 2004, a water control structure was used to restore a more natural water regime to a floodplain wetland: high winter and spring water levels (flooding some surveyed areas) followed by summer drawdown (exposure to natural tides). Over the previous 20 years, the water level had been artificially stabilized and reed canarygrass had invaded. Vegetation was surveyed around the edge of the wetland, in the autumn before (2003) and after (2004) the managed flood/drawdown. Plant species were recorded at 10 cm intervals along 27 transects (approximately 25,000 total points sampled), spanning areas with a tree canopy and areas of open marshland. The study does not generally separate results from the two habitat types.

    Study and other actions tested
  7. A study in 1999–2001 of a section of the Mississippi River in Illinois/Missouri, USA (Dugger & Feddersen 2009) reported that when the water level was successfully lowered for 30 days, herbaceous plant species colonized exposed parts of the floodplain. In 1999 and 2001, fifteen to seventeen plant taxa were recorded on the exposed floodplain. The most common taxa were sedges Cyperus spp. (occurring in 69–77% of surveyed quadrats), barnyard grasses Echinochloa spp. (53–80%) and knotweeds Polygonum spp. (28–93%). In 2000, low water levels could not be maintained for a continuous 30 day period. Vegetation that had germinated on exposed soils was killed by the floodwaters. Methods: Each spring between 1994 and 2001, downstream dam gates were opened to lower the water level in the focal section of the Mississippi River. The aim was to expose floodplain soils and allow certain herb species to germinate. Each year between 1999 and 2001, plant species and their cover were surveyed in 55–65 quadrats on the floodplain, approximately three weeks after initial exposure.

    Study and other actions tested
  8. A study in 1997–2006 of a levelled, irrigated and partially planted freshwater marsh in California, USA (Miller & Fujii 2010) reported that it developed vegetation dominated by emergent plants, including planted tule Schoenoplectus acutus – although vegetation cover and density depended on the water level. After 2–9 years, the shallower half of the site had 89–98% total vegetation cover. This included 77–81% cattail Typha spp., 11–19% tule and 0–5% submerged vegetation cover. Emergent vegetation density fluctuated between 49 and 76 stems/m2. The deeper half of the site had 77–100% total vegetation cover, including 38–58% cattail, 3–8% tule, and 10–46% submerged vegetation cover. Emergent vegetation density fluctuated between 44 and 59 stems/m2. Across the entire site, above-ground biomass of emergent vegetation was 1,630 g/m2 after 1–3 years (vs submerged, floating and algae combined: 389 g/m2) then fluctuated between 925 and 2,360 g/m2 for the following six years. Methods: In autumn 1997, a 0.6-ha area of farmland was levelled and lowered. Tule was planted into two 0.25-ha basins within the site. Shortly after planting, the fresh water was continuously piped into the site, flooding the basins with 25 cm and 55 cm of water respectively. The study does not distinguish between the effects of levelling, planting and irrigation on non-planted vegetation. All plants and algae were surveyed along transects, in summer/autumn, at least biennially between 1998 and 2006. Biomass was cut, dried and weighed (years 1–3) or estimated from plant height and diameter (years 4–9).

    Study and other actions tested
  9. A before-and-after study in 1984–1990 of two marshes on a floodplain in Florida, USA (Toth 2010) reported that following restoration of seasonal water level fluctuations, plant species richness and diversity decreased in both marshes, but the abundance of individual plant species responded differently in each marsh. Unless specified, statistical significance was not assessed. Plant species richness declined in both marshes, from 6.1–7.3 species/m2 in the year before intervention to 5.0–5.4 species/m2 after five years of fluctuating water levels – although this decline was only statistically significant in one marsh. Species diversity also declined in both marshes (data reported as a diversity index). The frequency of individual plant species, including those characteristic of broadleaf marshes, did not always respond consistently in both marshes. For example, pickerelweed Pontederia cordata frequency declined in one marsh (from 65% to 21% of quadrats) but was stable in the other (87% before and after). Methods: From September 1985, water level fluctuations were restored to two degraded marshes in Section B of the Kissimmee River floodplain, by actively managing the amount of water released into the river. Before intervention the marshes were nearly permanently flooded. After intervention, there were clear wet and dry seasons, with and without standing water. Plant species and their cover were surveyed before intervention (summer 1984–1985) and for five years after (until September 1990), in 1-m2 quadrats along two transects (37–48 quadrats/transect).

    Study and other actions tested
  10. A before-and-after study in 1992–2010 in historically drained marshes and wet meadows in Florida, USA (Sah et al. 2014) found that pumping water more or less directly into these drained areas generally led to development of more wetland-characteristic plant communities. All data were reported as graphical analyses. In management Phase 1 (1993–1999), water was pumped from a canal into part of the marsh. In one transect sampled before and at the end of this phase, the overall plant community became characteristic of wetter conditions in 80% of plots. Cover of some individual, wetland-characteristic plant species also increased in these plots. However, overall plant species richness declined. In management Phase 2 (2000–2010), water was pumped into storage basins upstream from the focal marsh. The basins gradually released the water as surface flow. During this phase, the overall plant community did not significantly change in most plots (77%) but became characteristic of drier conditions in the rest (23%). Methods: Vegetation in Taylor Slough was surveyed one year before and at the end of each water management phase. One transect was surveyed for Phase 1 and three transects for Phase 2. Plant species and cover were recorded in twenty 5-m2 permanent plots/transect/survey.

    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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