Raise water level to restore degraded brackish/salt marshes

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

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.


  • 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).


  • 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.


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 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
  2. 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
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

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

What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 21

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.

Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape ProgrammeRed List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Mauritian Wildlife Supporting Conservation Leaders
Sustainability Dashboard National Biodiversity Network Frog Life The international journey of Conservation - Oryx Cool Farm Alliance UNEP AWFA Bat Conservation InternationalPeople trust for endangered species Vincet Wildlife Trust