Raise water level to restore/create freshwater swamps from other land uses

Overall effectiveness category Unknown effectiveness (limited evidence)
Number of studies: 2
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How is the evidence assessed?

Effectiveness

40%
Certainty

32%
Harms

22%

Calculating overall effectiveness category

Background information and definitions

This action involves one-off action to raise the water level/table to restore/create swamps from other land uses. This means that intervention should (a) occur at one point in time, after which the water table is not actively managed, and (b) must affect an area that does not retain substantial characteristics of the target habitat. This could be an upland area (e.g. grassland), an unvegetated wetland (e.g. mudflats), or a wetland other than the target type (e.g. marsh, where the habitat used to be a swamp).

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 marsh/swamp); removing dams upstream of the focal marsh/swamp; 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. Rewetting drained acid sulphate soils – common in coastal areas and salinized inland areas – can lead to acidification, deoxygenation and release of toxic metals (Baldwin 2011).

Related actions: Raise water level to restore degraded swamps; Actively manage water level; 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.

^{Baldwin D. (2011) National Guidance for the Management of Acid Sulfate Soils in Inland Aquatic Ecosystems, Environment Protection and Heritage Council and the Natural Resource Management Ministerial Council, Australia.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of raising the water level to restore/create freshwater swamps from other land uses or habitat types. Both studies monitored the effects of one river dechannelization project in the USA.

VEGETATION COMMUNITY

Overall extent (1 study): One before-and-after study of a floodplain in the USA reported that after dechannelizing a river to raise the water level, the area of shrubby and forested wetlands increased – reaching greater coverage than before intervention, but also than before degradation.
Community types (1 study): The same study broke down overall swamp coverage into specific community types. For example, most of the shrubby wetlands that developed after raising the water level were dominated by a non-native species – which was not present historically.

VEGETATION ABUNDANCE

Overall abundance (1 study): One before-and-after, site comparison study of historical shrubby wetlands on a floodplain in the USA reported that dechannelizing a river to raise the water level reduced overall vegetation cover in the following nine years.
Characteristic plant abundance (1 study): The same study reported that after dechannelizing a river to raise the water level, only one of two sites became dominated by wetland-characteristic shrubs. The other site remained dominated by wetland-characteristic herb species.
Individual species abundance (1 study): The same study reported that dechannelizing a river to raise the water level slightly increased cover of buttonbush Cephalanthus occidentalis in one of two sites (no data for other site).

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

A before-and-after study in 1954–2008 in Florida, USA (Spencer & Bousquin 2014) reported that after dechannelizing a river to rewet the floodplain, the area of shrubby and forested wetlands increased. After roughly 3–8 years of rewetting, shrub-dominated wetlands covered 17–18% of the floodplain (vs 3–4% in a degraded state before rewetting, and 1% in the natural state before degradation). Most of the shrubby wetland area after rewetting was dominated by invasive Peruvian water primrose Ludwigia peruviana, which was not present before degradation. Mixed shrubby/herbaceous wetlands covered 7–18% of the floodplain after rewetting (vs 3–15% before rewetting and 52% before degradation). Coverage of forested wetlands was also greater after rewetting than before rewetting or degradation (data reported as maps). In total, wetland vegetation (shrubby, forested, herbaceous and submerged) covered 65–83% of the floodplain after rewetting (vs 22–37% before rewetting and 84% before degradation). Methods: Between 1999 and 2001, Section C of the Kissimmee River was dechannelized. This restored its natural meandering course, raised the water table on the adjacent floodplain and allowed for seasonal floods. Floodplain vegetation was mapped from aerial photographs taken before degradation (1954), during degradation (1974, 1996) and after restoration (2003, 2008). This study used the same rewetted floodplain section as (2).
Study and other actions tested
Referenced paper
Spencer L.J. & Bousquin S.G. (2014) Interim responses of floodplain wetland vegetation to Phase I of the Kissimmee River Restoration Project: comparisons of vegetation maps from five periods in the river's history. Restoration Ecology, 22, 397-408.
A before-and-after, site comparison study in 1998–2010 in Florida, USA (Toth 2017) reported that dechannelizing the river to rewet the floodplain had mixed effects on vegetation across two sites that were historically swamps. Statistical significance was not assessed. In the year before rewetting began, one restoration site (higher elevation) was dominated by shrubs: mostly upland (46–49% cover) but some wetland-characteristic (9% cover). The other restoration site (lower elevation) was dominated by wetland-characteristic herbs (71% cover). Total cover was 68–93%. Over roughly nine years after rewetting was complete, only the higher site had substantial cover of wetland-characteristic shrubs (3–29%). Canopy cover of habitat-characteristic buttonbush Cephalanthus occidentalis was <1–6% (vs before: 1%). The other site was dominated by wetland-characteristic herbs and floating/submerged plants. In both sites, vegetation cover after rewetting was highly variable across seasons and years (e.g. wetland-characteristic herbs: 1–82%; floating/submerged plants: 0–54%; overall: 1–92%). Over the entire study period, vegetation cover was relatively stable in another part of the floodplain that remained drained: a mixture of wetland and upland herbs (32–62% cover) and shrubs (8–34% cover). Methods: Between October 1999 and February 2001, Section C of the Kissimmee River floodplain was rewetted by dechannelizing the river. Eighteen 100-m² plots were established in parts of the floodplain that were historically buttonbush swamps (more recently drained and grazed/overgrown). There were 12 plots in the dechannelized section and six in an upstream section that remained channelized. Plant species and their cover were surveyed in spring and summer before intervention (1998–1999) and for roughly nine years after (until 2010). This study used the same rewetted floodplain section as (1).
Study and other actions tested
Referenced paper
Toth L.A. (2017) Variant restoration trajectories for wetland plant communities on a channelized floodplain. Restoration Ecology, 25, 342-353.

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.

Where has this evidence come from?

List of journals searched by synopsis

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

Related Actions

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Excavate brackish/saline pools Action Link	No evidence found (no assessment)	0	Synopsis Link
Reprofile/relandscape: brackish/saline swamps Action Link	Likely to be beneficial	5	Synopsis Link
Lower water level to restore/create brackish/saline swamps from other land uses Action Link	No evidence found (no assessment)	0	Synopsis Link