Lower water level to restore degraded freshwater marshes

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

Study locations

Key messages

  • Two studies evaluated the effects, on vegetation, of lowering the water level to restore degraded freshwater marshes. One study was in the USA and one was in Canada.


  • Overall extent (1 study): One before-and-after study of a lakeshore marsh in the USA reported that following a drawdown of water levels, emergent vegetation coverage increased in areas that were previously open water.
  • Community types (1 study): One replicated, controlled, before-and-after study of freshwater marshes in Canada reported changes in the area of some vegetation classes over three years of partial drawdown. There was a temporary increase in coverage of dead vegetation at the expense of some live vegetation classes. Two classes – horsetail-dominated and bur-reed-dominated – had greater coverage after three years of drawdown than before.




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 before-and-after study in 1993–1996 of a lakeshore marsh in Ohio, USA (Wilcox & Whillans 1999) reported that over a year of drawdown, the area of emergent vegetation increased. In 1993, two years before drawdown, the marsh was mostly open water with only 10% covered by emergent vegetation stands. In 1996, after approximately one year of drawdown but before any other interventions were carried out, 73% of the marsh was covered by emergent vegetation stands. Colonizing vegetation included several herbaceous wetland species, wind-dispersed woody species, and some upland herbs (not quantified). Methods: In 1995, an embankment was constructed across the mouth of Metzger Marsh to replace a natural barrier beach that had disappeared in the 1970s. The embankment separated the marsh from Lake Erie and caused a decline (drawdown) of the water level in the marsh. Vegetation was surveyed before (1993) and after (1996) drawdown (further details not reported).

    Study and other actions tested
  2. A replicated, controlled, before-and-after study in 2007–2010 of six freshwater marshes in Manitoba, Canada (Baschuk et al. 2012) found that a partial but long-term drawdown of the water level affected the area covered by some marsh vegetation classes. In marshes where the water level was drawn down, there was significant variation over time in the coverage of five vegetation classes. There was an initial decline followed by recovery in the area of vegetation dominated by sedges Carex spp. (before: 141 ha; after one year: 103 ha; after three years: 127 ha), cattails Typha spp. (111 ha; 69 ha; 98 ha) and horsetails Equisetum spp. (9 ha; 2 ha; 20 ha). Coverage of dead vegetation showed the opposite pattern (15 ha; 80 ha; 13 ha). Coverage of vegetation dominated by bur-reeds Sparganium spp. increased steadily over time (<1 ha; 2 ha; 7 ha). There was no significant change over time in the coverage of other vegetation classes, including trees (see original paper for data). In marshes that were not drawn down, the area of all vegetation classes was stable over time (see original paper for data). Methods: This study focused on six 84–207 ha marshes within one wetland complex. From August 2007, the water level was lowered in three marshes (average depth: 30 cm; maximum depth: 60 cm) using water control structures. This level was maintained over the entire study period. In the other three marshes, the water level remained unnaturally high (average: 67 cm; maximum: 100 cm). The coverage of vegetation types was measured from satellite images, taken in the summer before (2007) and for three years after (2008–2010) drawdown.

    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?

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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

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