Raise water level to restore/create brackish/salt marshes 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

45%
Certainty

30%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

This action involves one-off action to raise the water level/table to restore/create marshes 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. swamp, where the habitat used to be a marsh).

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 marshes; Flood cropland when fallow; 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 brackish/salt marshes from other land uses or habitat types. Both studies were in the same area of Iraq, but used different study sites.

VEGETATION COMMUNITY

Community types (1 study): One before-and-after study of a slightly brackish marsh in Iraq reported that fewer plant community types were present three years after reflooding than before drainage.
Overall richness/diversity (2 studies): Two before-and-after studies of brackish marshes in Iraq reported that fewer plant species were present three years after reflooding than before drainage. One of these studies also reported that individual plant communities typically had lower diversity after reflooding than before drainage.

VEGETATION ABUNDANCE

Overall abundance (1 study): One before-and-after study of a slightly brackish marsh in Iraq reported that six of seven studied plant communities had lower spring and/or summer biomass three years after reflooding than before drainage.

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 replicated, before-and-after study in the early 1990s and 2006 of two brackish marshes in southern Iraq (Al-Abbawy & Al-Mayah 2010) reported that after reflooding, the marshes contained fewer plant species than they contained before drainage. Approximately three years after reflooding, 24–27 plant species were recorded in each marsh (10–12 emergent, 12 submerged, 2–3 floating). Before drainage, 38–44 plant species were recorded in each marsh (18–19 emergent, 12–16 submerged, 8–9 floating). Methods: Monthly surveys were carried out between January 2006 and December 2007 to record plant species in Central Marsh (two sites) and East Hammar Marsh (two sites). These brackish marshes were drained in the 1990s – becoming “almost totally desiccated” by 2000, but retaining small pockets of remnant marsh vegetation. The marshes were reflooded from 2003 (details not reported). Previously published data from the same marshes, collected in the early 1990s before drainage, were used for comparison.
Study and other actions tested
Referenced paper
Al-Abbawy D.A.H. & Al-Mayah A.A. (2010) Ecological survey of aquatic macrophytes in restored marshes of Southern Iraq during 2006 and 2007. Marsh Bulletin, 5, 177-196.
A before-and-after study in 1973–2006 of a brackish marsh in southern Iraq (Hamdan et al. 2010) reported that after reflooding, the marsh contained fewer plant species and communities than before it was drained, and that those communities typically had lower diversity and biomass. Statistical significance was not assessed. Within three years of reflooding, 38 plant species were recorded in the marsh (vs 48 before drainage). Twenty-six species were present both before and after reflooding. Three years after reflooding, 10 distinct plant communities were recorded in the marsh (vs 14 before drainage). For six of seven communities with comparable data, plant diversity was lower after reflooding than before drainage (data reported as a diversity index). Results for above-ground vegetation biomass were more mixed and depended on the season of comparison, but for six communities biomass was lower after reflooding than before drainage in at least one season (for which after: 50–3,247 g/m²; before: 60–4,923 g/m²). Methods: In 2003, local residents released water from canals and reservoirs to reflood marshes on the Mesopotamian Plain that had been almost completely drained in the 1990s. In spring and summer 2006, vegetation was surveyed in three sites within the slightly brackish (salinity 1–2 ppt) reflooded Central Marsh. Species, cover and biomass were recorded/collected in seven hundred 1-m²quadrats. Biomass was later dried and weighed. Previously published data from the 1970s (from different sites within the marsh) were used for comparison.
Study and other actions tested
Referenced paper
Hamdan M.A., Asada T., Hassan F.M., Warner B.G., Douabul A., Al-Hilli M.R.A. & Alwan A.A. (2010) Vegetation response to re-flooding in the Mesopotamian Wetlands, southern Iraq. Wetlands, 30, 177-188.

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?

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This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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