Actively manage water level: freshwater swamps

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

Effectiveness

60%
Certainty

40%
Harms

10%

Calculating overall effectiveness category

Background information and definitions

This action involves active, repeated management of the amount of water in wetlands and when it is present, to mimic the natural hydrology of swamps. This may prevent excessively high or low water levels (e.g. during storm surges or droughts), or maintain wet/dry cycles that are a natural feature of many swamps (e.g. Conner & Buford 1998).

This action will usually involve some kind of water control structure: a valve, gate, sluice or pump. Water level management might aim to: mirror historical water level fluctuations or stability; manage salinity levels; increase sediment inputs; stimulate growth of desirable plant species; and/or create new wetland plant communities. Studies of “moist soil management”, “structural marsh management” and “environmental flows” along river courses all fall within the scope of this action. Caution: When managing water levels in a focal site, the effect on water levels in neighbouring sites should be considered.

Although water levels may be managed to restore or enhance habitats for waterfowl, information on the value of vegetation for waterfowl (e.g. seed production; productivity measured as CO₂ exchange rates) is not summarized in this synopsis. Also, this synopsis does not include information on riparian areas that are not clearly marshes or swamps (e.g. riparian forests that require only a brief flood pulse for germination; Taylor et al. 2006).

^{Conner W.H. & Buford M.A. (1998) Southern deepwater swamps. Pages 261–287 in: M.G. Messina & W.H. Conner (eds.) Southern Forested Wetlands: Ecology and Management. Lewis Publishers/CRC Press, Boca Raton.}

^{Junk W.J., Piedade M.T.F., Schöngart J., Cohn-Haft M., Adeney J.M. Wittman F. (2011) A classification of major naturally-occurring Amazonian lowland wetlands. Wetlands, 31, 623–640.}

^{Taylor J.P., Smith L.M. & Haukos D.A. (2006) Evaluation of woody plant restoration in the Middle Rio Grande: ten years after. Wetlands, 26, 1151–1160.}

Study locations

Key messages

Two studies evaluated the effects, on vegetation, of active water level management in freshwater swamps. Both studies were in the USA.

VEGETATION COMMUNITY

Overall richness/diversity (1 study): One before-and-after study of a swamp/marsh in the USA found that overall plant diversity was higher in the autumn following a managed flood/drawdown than in the autumn before.

VEGETATION ABUNDANCE

Tree/shrub abundance (1 study): One site comparison study of floodplain swamps in the USA found that an artificial flood had no significant effect on tree seedling density in a low and very wet swamp, but increased tree seedling density in a drier swamp higher on the floodplain.
Individual species abundance (1 study): One before-and-after study of a swamp/marsh in the USA reported mixed responses of individual plant species’ cover to active water level management. However, the study found that cover of the dominant woody species, Pacific willow Salix lucida, was higher in the autumn following a managed flood/drawdown than in the autumn before.

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 2003–2004 of a freshwater wetland with marsh and swamp vegetation in Oregon, USA (Jenkins et al. 2008) found that following a managed flood/drawdown, plant diversity increased and there were changes in cover of individual plant taxa. Plant diversity was higher in the autumn after the flood/drawdown than in the autumn before (data reported as a diversity index). Of 21 plant taxa for which cover data were reported, 12 became more abundant, including knotweeds Polygonum spp. (before: 21%; after: 35%) and Pacific willow Salix lucida (before: 11%; after: 15%). The cover of seven taxa declined, including invasive reed canarygrass Phalaris arundinacea (before: 44%; after: 41%). The largest canarygrass declines occurred under regenerating tree canopies and in areas more deeply flooded during spring 2004 (see original paper for data). Methods: In 2004, a water control structure was used to restore a more natural water regime to a floodplain wetland: high winter and spring water levels (flooding some surveyed areas) followed by summer drawdown (exposure to natural tides). Over the previous 20 years, the water level had been artificially stabilized and reed canarygrass had invaded. Vegetation was surveyed around the edge of the wetland, in the autumn before (2003) and after (2004) the managed flood/drawdown. Plant species were recorded at 10 cm intervals along 27 transects (approximately 25,000 total points sampled), spanning marshy areas (open, herbaceous) and swampy areas (with a tree canopy). The study does not generally separate results from the two habitat types.
Study and other actions tested
Referenced paper
Jenkins N.J., Yeakley J.A. & Stewart E.M. (2008) First-year responses to managed flooding of lower Columbia River bottomland vegetation dominated by Phalaris arundinacea. Wetlands, 28, 1018-1027.
A site comparison study in 2006–2007 of two forested floodplains in South Carolina and Georgia, USA (Lee et al. 2016) found that an artificial flood pulse increased the number of tree seedlings in one of two forest types, but had no significant effect in the other. No data were reported for these results. The Savannah River floodplain was artificially flooded in spring 2006 by releasing water from an upstream reservoir, but was not flooded in spring 2007. In cypress-tupelo swamp forest, the number of tree seedlings/plot did not significantly differ between years. In bottomland hardwood forest (higher up on the floodplain), the number of tree seedlings/plot was greater in summer 2006 than summer 2007. The nearby Altamaha River floodplain experienced natural floods in spring 2006 and 2007. Here, the overall number of tree seedlings/plot did not significantly differ between years for both forest types. Methods: Tree seedlings were counted in July–September 2006 and 2007, in around 50 permanent 30-m² plots/river/year.
Study and other actions tested
Referenced paper
Lee L.S., Garnett J.A., Bright E.G., Sharitz R.R. & Batzer D.P. (2016) Vegetation, invertebrate, and fish community response to past and current flow regulation in floodplains of the Savannah River, southeastern USA. Wetlands Ecology and Management, 24, 443-455.

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

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