Raise water level to restore degraded brackish/salt marshes

Overall effectiveness category Likely to be beneficial
Number of studies: 2
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How is the evidence assessed?

Effectiveness

60%
Certainty

46%
Harms

15%

Calculating overall effectiveness category

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.

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of raising the water level to restore degraded brackish/salt marshes. One study was in the Netherlands and one was in Tunisia.

VEGETATION COMMUNITY

Community types (2 study): One before-and-after study of a lakeshore brackish/salt marsh in Tunisia reported an increase in coverage of bulrush-dominated vegetation relative to salt marsh vegetation over three years after modifying a canal to retain water in the marsh. One study of a salt marsh in the Netherlands reported increased coverage of pioneer succulent plant communities, and reduced coverage of short-grass communities, over approximately 10 years following abandonment of the drainage system (along with other interventions).
Overall richness/diversity (1 study): One study of a salt marsh in the Netherlands reported that overall plant species richness increased over 14 years after abandoning drainage systems (along with other interventions).

VEGETATION ABUNDANCE

Individual species abundance (1 study): One study of a salt marsh in the Netherlands reported that some individual plant species became more common over 14 years after abandoning drainage systems (along with other interventions). These included saltbush Atriplex prostrata and seablite Suaeda maritima. Some other species became less common, including creeping bentgrass Agrostis stolonifera and common cordgrass Spartina anglica.

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 study in 1981–1997 of a salt marsh in the Netherlands (Esselink et al. 2002) found that following abandonment of drainage systems from 1981 (along with legal protection and a reduction in grazing intensity), there were changes in the area of plant community types and the abundance of some dominant species, and an increase in plant species richness. Between 1981 and 1995, the area covered by pioneer succulents increased (from 0% to 19% of the marsh) and the area covered by short-grass communities decreased (from 76% to 56%). Statistical significance of these cover results was not assessed. Between 1983 and 1997, the frequency of two of the most abundant plant species did not significantly change: saltmarsh grass Puccinellia maritima (1983: present in 81% of plots; 1997: present in 84% of plots) and sea aster Aster tripolium (1983: 80%; 1997: 97%). Species showing significant changes in frequency included saltbush Atriplex prostrata (increase from 86% to 98%), seablite Suaeda maritima (increase from 38% to 70%), creeping bentgrass Agrostis stolonifera (decrease from 78% to 69%) and common cordgrass Spartina anglica (decrease from 52% to 31%). Between 1983 and 1997, plant species richness significantly increased: from 8 species/100 m² to 10 species/100 m². Methods: A degraded coastal salt marsh became part of a nature reserve in 1981. The drainage system was abandoned by 1984 (making the soils wetter and less aerated), and cattle grazing intensity was gradually reduced (reaching 40–80 animal days/ha/season by the 1990s). Note that this study evaluates the combined effect of these interventions. Coverage of vegetation types was calculated from maps of the marsh made in 1981 and 1995. Plant species presence and cover were surveyed in 64 permanent 100-m² plots, spread across four parts of the marsh and at a range of elevations, in 1983, 1991 and 1997.
Study and other actions tested
Referenced paper
Esselink P., Fresco L.F.M. & Dijkema K.S. (2002) Vegetation change in a man‐made salt marsh affected by a reduction in both grazing and drainage. Applied Vegetation Science, 5, 17-32.
A before-and-after study in 2005–2011 of a lakeshore brackish/saline marsh in Tunisia (Ouali et al. 2014) reported that after building dams and embankments along a canal to raise the water level in the marsh, the ratio of bulrush-dominated vegetation to salt-marsh vegetation increased. In 2005, after three years of freshwater releases from upstream dams to restore winter flooding, the marsh contained 241 ha of vegetation dominated by bulrush Bolboschoenus glaucus, and 468 ha of salt marsh communities (dominated by glasswort Sarcocornia fruticosa and sea barley Hordeum marinum) – a ratio of 0.5:1. Between 2008 and 2011, after modifying a canal within the marsh to hold back water (dams inserted, and arrays of embankments built perpendicular to the canal) along with continued freshwater releases, the area of bulrush-dominated vegetation increased from 0.5:1 (193:399 ha) to 1:1 (298:296 ha). Methods: Between 2005 and 2011, vegetation in the lakeshore Joumine Marsh was mapped using field surveys and satellite images.
Study and other actions tested
Referenced paper
Ouali M., Daoud-Bouattour A., Etteieb S., Gammar A.M., Ben Saad-Limam S. & Ghrabi-Gammar Z. (2014) Joumine Marsh, National Park of Ichkeul, Tunisia: floristic diversity, vegetation mapping and dynamics (1925–2011). Revue d'Écologie (La Terre et La Vie), 69, 3-23.

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

All the journals searched for all synopses

This Action forms part of the Action Synopsis:

Marsh and Swamp Conservation

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Raise water level to restore degraded freshwater marshes Action Link	Trade-off between benefit and harms	5	Synopsis Link